Multi-clock PHY preamble design and detection

ABSTRACT

In a method for detecting a clock rate of a data unit, the data unit is received via a communication channel. The data unit includes i) a first preamble portion and ii) an orthogonal frequency division multiplexing (OFDM) portion following the first preamble portion. The OFDM portion includes a second preamble portion including one or more long training fields. Based on the first preamble portion, whether a clock rate of the OFDM portion is a first clock rate or a second clock rate lower than the first clock rate is determined based on the first preamble portion.

CROSS-REFERENCE TO RELATED APPLICATION

This disclosure claims the benefit of U.S. Provisional PatentApplication No. 61/441,610, filed on Feb. 10, 2011, the disclosure ofwhich is hereby incorporated by reference herein in its entirety.

This application is also related to the commonly-owned, co-pendingpatent application U.S. patent application Ser. No. 13/365,950, entitled“Multi-Clock PHY Preamble Design and Detection”, filed on the same dayas the present application and hereby incorporated by reference hereinin its entirety.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to communication networks and,more particularly, to wireless local area networks including physicallayer modes with multiple clock rates.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

When operating in an infrastructure mode, wireless local area networks(WLANs) typically include an access point (AP) and one or more clientstations. WLANs have evolved rapidly over the past decade. Developmentof WLAN standards such as the Institute for Electrical and ElectronicsEngineers (IEEE) 802.11a, 802.11b, 802.11g, and 802.11n Standards hasimproved single-user peak data throughput. For example, the IEEE 802.11bStandard specifies a single-user peak throughput of 11 megabits persecond (Mbps), the IEEE 802.11a and 802.11g Standards specify asingle-user peak throughput of 54 Mbps, the IEEE 802.11n Standardspecifies a single-user peak throughput of 600 Mbps, and the IEEE802.11ac Standard specifies a single-user peak throughput in the Gbpsrange.

Work has begun on two new standards, IEEE 802.11ah and IEEE 802.11af,each of which will specify wireless network operation in sub-1 GHzfrequencies. Low frequency communication channels are generallycharacterized by better propagation qualities and extended propagationranges compared to higher frequency communication channels. In the past,sub-1 GHz frequency ranges have not been utilized for wirelesscommunication networks because such frequencies were reserved for otherapplications (e.g., licensed TV frequency bands, radio frequency band,etc.). There are few frequency bands in the sub-1 GHz range that remainunlicensed, with different unlicensed frequencies in differentgeographical regions. The IEEE 802.11ah Standard will specify wirelessoperation in available unlicensed sub-1 GHz frequency bands. The IEEE802.11af Standard will specify wireless operation in TV White Space(TVWS), i.e., unused TV channels in sub-1 GHz frequency bands.

SUMMARY

In an embodiment, a method for detecting a clock rate of a data unitincludes receiving the data unit via a communication channel. The dataunit includes i) a first preamble portion and ii) an orthogonalfrequency division multiplexing (OFDM) portion following the firstpreamble portion. The OFDM portion includes a second preamble portionincluding one or more long training fields. The method also includesdetermining, based on the first preamble portion, whether a clock rateof the OFDM portion is a first clock rate or a second clock rate lowerthan the first clock rate.

In another embodiment, a communication device includes a networkinterface configured to receive a data unit via a communication channel.The data unit includes i) a first preamble portion and ii) an orthogonalfrequency division multiplexing (OFDM) portion following the firstpreamble portion. The OFDM portion includes a second preamble portionincluding one or more long training fields. The network interface isalso configured to determine, based on the first preamble portion,whether a clock rate of the OFDM portion is a first clock rate or asecond clock rate lower than the first clock rate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an example wireless local area network(WLAN), according to an embodiment.

FIG. 2 is a diagram of a prior art orthogonal frequency divisionmultiplexing (OFDM) short range data unit that an AP and/or clientstation is configured to transmit via a communication channel, accordingto an embodiment.

FIG. 3 is a diagram of a prior art OFDM short range data unit that an APand/or client station is configured to transmit via a communicationchannel, according to another embodiment.

FIG. 4 is a diagram of a prior art OFDM short range data unit that an APand/or client station is configured to transmit via a communicationchannel, according to another embodiment.

FIG. 5 is a diagram of a prior art OFDM short range data unit that an APand/or client station is configured to transmit via a communicationchannel, according to another embodiment.

FIG. 6 is a diagram of a prior art single carrier (SC) short range dataunit that an AP and/or client station is configured to transmit via acommunication channel, according to an embodiment.

FIG. 7 is a diagram of a first example preamble design and correspondingauto-detection technique for determining a data unit clock rate,according to an embodiment.

FIG. 8 is a diagram of a second example preamble design andcorresponding auto-detection technique for determining a data unit clockrate, according to an embodiment.

FIG. 9 is a diagram of a third example preamble design and correspondingauto-detection technique for determining a data unit clock rate,according to an embodiment.

FIG. 10 is a diagram of a fourth example preamble design correspondingto an auto-detection technique for determining a data unit clock rate,according to an embodiment.

FIG. 11 is a diagram of a fifth example preamble design corresponding toan auto-detection technique for determining a data unit clock rate,according to an embodiment.

FIG. 12 is a diagram of a sixth example preamble design corresponding toan auto-detection technique for determining a data unit clock rate,according to an embodiment.

FIG. 13 is a flow diagram of an example method for generating a dataunit according to the first, second, third, fourth, fifth, or sixthexample preamble design, according to an embodiment.

FIG. 14 is a flow diagram of an example method for auto-detecting aclock rate of a data unit generated according to the first, second,third, fourth, fifth, or sixth example preamble design, according to anembodiment.

FIG. 15 is a flow diagram of an example method for generating a dataunit according to the first example preamble design, according to anembodiment.

FIG. 16 is a flow diagram of an example method for auto-detecting aclock rate of a data unit generated according to the first examplepreamble design, according to an embodiment.

FIG. 17 is a flow diagram of an example method for generating a dataunit according to the second example preamble design, according to anembodiment.

FIG. 18 is a flow diagram of an example method for auto-detecting aclock rate of a data unit generated according to the second examplepreamble design, according to an embodiment.

FIG. 19 is a flow diagram of an example method for generating a dataunit according to the third example preamble design, according to anembodiment.

FIG. 20 is a flow diagram of an example method for auto-detecting aclock rate of a data unit generated according to the third examplepreamble design, according to an embodiment.

FIG. 21 is a flow diagram of an example method for generating a dataunit according to the fourth or fifth example preamble design, accordingto an embodiment.

FIG. 22 is a flow diagram of an example method for auto-detecting aclock rate of a data unit generated according to the fourth or fifthexample preamble design, according to an embodiment.

FIG. 23 is a flow diagram of an example method for generating a dataunit according to the sixth example preamble design, according to anembodiment.

FIG. 24 is a flow diagram of an example method for auto-detecting aclock rate of a data unit generated according to the sixth examplepreamble design, according to an embodiment.

DETAILED DESCRIPTION

In embodiments described below, a wireless network device such as anaccess point (AP) of a wireless local area network (WLAN) transmits datastreams to, and/or receives data streams from, one or more clientstations. The AP is configured to communicate with client stationsaccording to at least a first communication protocol. The firstcommunication protocol defines operation in a sub-1 GHz frequency range,and is typically used for applications requiring longer range wirelesscommunication (as compared to WLAN systems that conform to the IEEE802.11a, 802.11b, 802.11g, and 802.11n Standards) with relatively lowdata rates (as compared to WLAN systems that conform to the IEEE802.11a, 802.11b, 802.11g, and 802.11n Standards). The firstcommunication protocol (e.g., IEEE 802.11af or IEEE 802.11ah) isreferred to herein as a “long range” communication protocol. In someembodiments, the AP is also configured to communicate with clientstations according to one or more other communication protocols whichdefine operation in generally higher frequency ranges and are typicallyused for closer-range communications with higher data rates. The higherfrequency communication protocols (e.g., IEEE 802.11a, IEEE 802.11n,and/or IEEE 802.11ac) are collectively referred to herein as “shortrange” communication protocols.

In some embodiments, the physical layer (PHY) data units conforming tothe long range communication protocol (“long range data units”) are thesame as or similar to data units conforming to a short rangecommunication protocol (“short range data units”), but are generatedusing a lower clock rate. For example, in an embodiment, a device (e.g.,an AP) generates along range data unit by down-sampling or“down-clocking” a clock rate used to generate the short range dataunits. Accordingly, in some embodiments, a single communication deviceis capable of generating multiple types of data units (e.g., long rangeand short range data units), each type having a similar format butgenerated using a different clock rate. Thus, in some embodiments, twoor more differently clocked data units corresponding to two or moredifferent PHY modes coexist at the same time in the same region. In someembodiments, a single WLAN includes communications conforming to two ormore long range communication modes, each utilizing data units that aredown-clocked from a short range data unit (e.g., first and second PHYmodes down-clocked to ¼ and ⅛, respectively, of the IEEE 802.11n dataunit clock rate, in an embodiment).

All else being equal, an orthogonal frequency division multiplexing(OFDM) symbol generated using a faster clock is shorter in duration thanan OFDM symbol generated using a slower clock. To properly demodulatereceived data units that include OFDM symbols (e.g., IEEE 802.11a, IEEE802.11n, IEEE 802.11ac, IEEE 802.11af, and IEEE 802.11ah data units), areceiving device generally must know the clock rate that thetransmitting device used to generate the received data unit.Accordingly, where different clock rates are used for different PHYmodes in a single region and at the same time, communication deviceswithout a priori knowledge must determine or auto-detect the clock rateof received data units. Various embodiments of data unit preambledesigns, and corresponding receiver techniques for auto-detecting clockrates based on the preamble designs, are disclosed herein.

FIG. 1 is a block diagram of an example wireless local area network(WLAN) 10, according to an embodiment. An AP 14 includes a hostprocessor 15 coupled to a network interface 16. The network interface 16includes a medium access control (MAC) unit 18 and a PHY unit 20. ThePHY unit 20 includes a plurality of transceivers 21, and thetransceivers are coupled to a plurality of antennas 24. Although threetransceivers 21 and three antennas 24 are illustrated in FIG. 1, the AP14 can include different numbers (e.g., 1, 2, 4, 5, etc.) oftransceivers 21 and antennas 24 in other embodiments.

The WLAN 10 includes a plurality of client stations 25. Although fourclient stations 25 are illustrated in FIG. 1, the WLAN 10 can includedifferent numbers (e.g., 1, 2, 3, 5, 6, etc.) of client stations 25 invarious scenarios and embodiments. The client station 25-1 includes ahost processor 26 coupled to a network interface 27. The networkinterface 27 includes a MAC unit 28 and a PHY unit 29. The PHY unit 29includes a plurality of transceivers 30, and the transceivers 30 arecoupled to a plurality of antennas 34. Although three transceivers 30and three antennas 34 are illustrated in FIG. 1, the client station 25-1can include different numbers (e.g., 1, 2, 4, 5, etc.) of transceivers30 and antennas 34 in other embodiments. In an embodiment, one, two, orthree of the client stations 25-2, 25-3, and 25-4 have a structure thesame as or similar to the client station 25-1. In these embodiments, theclient stations 25 are structured the same as or similar to the clientstation 25-1 and have the same or a different number of transceivers andantennas. For example, the client station 25-2 has only two transceiversand two antennas, in an embodiment.

In various embodiments, the PHY unit 20 of the AP 14 is configured tooperate in any of multiple PHY modes. In some embodiments, each PHY modecorresponds to a particular communication protocol, or to a particularmode of a communication protocol. As a result, in some embodiments, eachPHY mode corresponds to using a particular clock rate to generatecorresponding data units. For example, a first PHY mode corresponds to ashort range communication protocol for which the PHY unit 20 generatesdata units using a first clock rate, and a second PHY mode correspondsto a long range communication protocol for which the PHY unit 20generates data units using a second clock rate that is down-clocked fromthe first clock rate, in an embodiment. As another example, a first PHYmode corresponds to a short range communication protocol for which thePHY unit 20 generates data units using a first clock rate, a second PHYmode corresponds to a “regular” mode of a long range communicationprotocol for which the PHY unit 20 generates data units using a secondclock rate that is down-clocked from the first clock rate (e.g., ¼ thefirst clock rate), and a third PHY mode corresponds to an “extendedrange” mode of the long range communication protocol for which the PHYunit 20 generates data units using a third clock rate that is furtherdown-clocked from the first clock rate (e.g., ⅛ the first clock rate),in an embodiment.

The transceiver(s) 21 of the AP 14 is/are configured to transmit thegenerated data units via the antenna(s) 24. Similarly, thetransceiver(s) 21 is/are configured to receive similar data units viathe antenna(s) 24. In various embodiments, the PHY unit 20 of the AP 14is further configured to process received data units (e.g., data unitsthat conform to any of the communication protocols and PHY modes thatthe PHY unit 20 supports for transmission).

In some embodiments, the PHY unit 29 of the client station 25-1 isconfigured to generate data units conforming to only a single PHY modethat corresponds to a particular communication protocol and data unitclock rate. In other embodiments, the PHY unit 29, in a manner similarto the PHY unit 20, is configured to generate data units conforming toany of multiple PHY modes, with each PHY mode corresponding to aparticular communication protocol (or a particular mode of acommunication protocol) and a particular data unit clock rate.

The transceiver(s) 30 is/are configured to transmit the generated dataunits via the antenna(s) 34. Similarly, the transceiver(s) 30 is/areconfigured to receive data units via the antenna(s) 34. The PHY unit 29of the client station 25-1 is further configured to process receiveddata units (e.g., data units that conform to any of the communicationprotocols and PHY modes that the PHY unit 29 supports for transmission).

Similar to client station 25-1, each of client stations 25-2, 25-3, and25-4 is configured to transmit and/or receive data units correspondingto only a single PHY mode, or data units corresponding to any one ofmultiple PHY modes, in various embodiments. In some embodiments, one ormore of the client stations 25-1 through 25-4 is configured to transmitand/or receive data units corresponding to a PHY mode that is notsupported by one or more others of the client stations 25-1 through25-4. For example, in an embodiment, the client station 25-1 isconfigured to transmit and/or receive only short range data unitsclocked at a first rate, while the client station 25-2 is configured totransmit and/or receive only long range data units clocked at a second,slower rate.

FIG. 2 is a diagram of a prior art OFDM short range data unit 100 thatan AP (e.g., AP 14 of FIG. 1) and/or a client station (e.g., clientstation 25-1 of FIG. 1) is configured to transmit via a communicationchannel, according to an embodiment. The data unit 100 conforms to theIEEE 802.11a Standard and occupies a 20 Megahertz (MHz) band. The dataunit 100 includes a preamble having a legacy short training field(L-STF) 102, generally used for packet detection, initialsynchronization, and automatic gain control, etc., and a legacy longtraining field (L-LTF) 104, generally used for channel estimation andfine synchronization. The data unit 100 also includes a legacy signalfield (L-SIG) 106, used to carry certain PHY parameters of the data unit100 such as the modulation type and coding rate used to generate thedata unit 100, for example. The data unit 100 also includes a dataportion 108. According to some embodiments and/or scenarios, the dataportion 108 includes a service field, a scrambled PHY service data unit(PSDU), tail bits, and padding bits, if needed. The data unit 100 isdesigned for transmission over one spatial or space-time stream in asingle input single output (SISO) channel configuration.

FIG. 3 is a diagram of a prior art OFDM short range data unit 120 thatan AP (e.g., AP 14 of FIG. 1) and/or a client station (e.g., clientstation 25-1 of FIG. 1) is configured to transmit via a communicationchannel, according to another embodiment. The data unit 120 conforms tothe IEEE 802.11n Standard, occupies a 20 MHz band, and corresponds to a“mixed” mode designed for scenarios where the WLAN includes both clientstations that conform to the IEEE 802.11n Standard and client stationsthat conform to the IEEE 802.11a Standard but not the IEEE 802.11nStandard. The data unit 120 includes a preamble having an L-STF 122, anL-LTF 124, an L-SIG 126, a high throughput signal field (HT-SIG) 128, ahigh throughput short training field (HT-STF) 130, and M high throughputlong training fields (HT-LTFs) 132-1 through 132-M, where M is aninteger which generally corresponds to a number of spatial streams usedto transmit the data unit 120 in a multiple input multiple output (MIMO)channel configuration. In particular, according to the IEEE 802.11nStandard, the data unit 120 includes two HT-LTFs 132 if the data unit120 is transmitted using two spatial streams, and four HT-LTFs 132 ifthe data unit 120 is transmitted using three or four spatial streams. Anindication of the particular number of spatial streams being utilized isincluded in the HT-SIG 128. The data unit 120 also includes a highthroughput data portion (HT-DATA) 134.

Within the data unit 120, L-SIG 126 is modulated according to binaryphase shift keying (BPSK), while HT-SIG 128 is modulated according toBPSK on the quadrature axis (Q-BPSK). In other words, the modulation ofHT-SIG 128 is rotated by 90 degrees as compared to the modulation ofL-SIG 126. Such modulation allows a receiving device to determine orauto-detect, without decoding the entire preamble, that the data unit120 conforms to the IEEE802.11n Standard rather than the IEEE 802.11aStandard.

FIG. 4 is a diagram of a prior art OFDM short range data unit 140 thatan AP (e.g., AP 14 of FIG. 1) and/or a client station (e.g., clientstation 25-1 of FIG. 1) is configured to transmit via a communicationchannel, according to another embodiment. The data unit 140 conforms tothe IEEE 802.11n Standard, occupies a 20 MHz band, and corresponds to a“Greenfield” mode designed for scenarios where the WLAN does not includeany client stations that conform to the IEEE 802.11a Standard but notthe IEEE 802.11n Standard. The data unit 140 includes a preamble havinga high throughput Greenfield short training field (HT-GF-STF) 142, afirst high throughput long training field (HT-LTF1) 144, a HT-SIG 146,and M HT-LTFs 148-1 through 148-M, where M is an integer which generallycorresponds to a number of spatial streams used to transmit the dataunit 140 in a MIMO channel configuration. The data unit 140 alsoincludes a data portion 150.

FIG. 5 is a diagram of a prior art OFDM short range data unit 170 thatan AP (e.g., AP 14 of FIG. 1) and/or a client station (e.g., clientstation 25-1 of FIG. 1) is configured to transmit via a communicationchannel, according to another embodiment. The data unit 170 conforms tothe IEEE 802.11ac Standard and is designed for scenarios where the WLANincludes both client stations that conform to the IEEE 802.11ac Standardand client stations that conform to the IEEE 802.11a Standard but notthe IEEE 802.11ac Standard. The data unit 170 occupies a 20 MHzbandwidth. In other embodiments or scenarios, a data unit similar to thedata unit 170 occupies a different bandwidth, such as a 40 MHz, an 80MHz, or a 160 MHz bandwidth. The data unit 170 includes a preamblehaving an L-STF 172, an L-LTF 174, an L-SIG 176, a first very highthroughput signal field (VHT-SIG-A) 178, a very high throughput shorttraining field (VHT-STF) 180, M very high throughput long trainingfields (VHT-LTFs) 182-1 through 182-M, where M is an integer, and asecond very high throughput signal field (VHT-SIG-B) 184. The data unit170 also includes a very high throughput data portion (VHT-DATA) 186. Insome embodiments, the data unit 170 is a multi-user data unittransmitted by an AP (e.g., AP 14 of FIG. 1) which carries informationto more than one client station (e.g., one or more of the clientstations 25 of FIG. 1) simultaneously. In such embodiments or scenarios,VHT-SIG-A 178 includes information common to all of the intended clientstations, and VHT-SIG-B 184 includes user-specific information for eachof the intended client stations.

Within the data unit 170, L-SIG 176 and VHT-SIG-A 178 are modulatedaccording to BPSK, while VHT-SIG-B 184 is modulated according to Q-BPSK.Similar to the IEEE 802.11n auto-detection feature discussed above, suchmodulation allows a receiving device to determine or auto-detect,without decoding the entire preamble, that the data unit 170 conforms tothe IEEE802.11ac Standard rather than the IEEE 802.11a Standard.

FIG. 6 is a diagram of a prior art single carrier (SC) short range dataunit 200 that an AP (e.g., AP 14 of FIG. 1) and/or a client station(e.g., client station 25-1 of FIG. 1) is configured to transmit via acommunication channel, according to another embodiment. The data unit200 conforms to the IEEE 802.11b Standard, and is modulated by directsequence spread spectrum (DSSS) or complementary code keying (CCK), invarious embodiments. The data unit 200 includes a synchronization (SYNC)field 202 that allows a receiver to detect the presence of the data unit200 and begin synchronizing with the incoming signal. The data unit 200also includes a start frame delimiter (SFD) field 204 that signals thebeginning of a frame. SYNC field 202 and SFD field 204 form the preambleportion of the data unit 200. The data unit 200 also includes a headerportion containing a signal field 206, a service field 808, a lengthfield 210, and a cyclic check redundancy check (CRC) field 212. The dataunit 200 also includes a PHY service data unit (PSDU) 214, i.e., thedata portion.

In various embodiments and/or scenarios, data units that conform to along range communication protocol (e.g., the IEEE 802.11af or 802.11ahStandard) are formatted at least substantially the same as defined bythe IEEE 802.11a Standard, the 802.11n Standard (mixed mode orGreenfield), or the 802.11ac Standard, as described and shown above inconnection with FIGS. 2-5, but are transmitted at a lower frequency(e.g., sub-1 GHz) and using a slower clock rate. In some suchembodiments, a transmitting device (e.g., the AP 14) down-clocks by afactor of N the clock rate used for generating the short range dataunits, to a lower clock rate to be used for generating the long rangedata units. The long range data unit is therefore generally transmittedover a longer time, and optionally occupies a smaller bandwidth, thanthe corresponding short range data unit. The down-clocking factor N isdifferent according to different embodiments and/or scenarios. In oneembodiment, the down-clocking factor N is equal to 10. In otherembodiments, other suitable down-clocking factor (N) values areutilized, and transmission times and bandwidths of long range data unitsare scaled accordingly. In some embodiments, the down-clocking factor Nis a power of two (e.g., N=8, 16, 32, etc.).

Examples of long range data units generated by down-clocking aredescribed in U.S. patent application Ser. No. 13/359,336, filed Jan. 26,2012, which is hereby incorporated by reference herein in its entirety.In some embodiments, and also as described in U.S. patent applicationSer. No. 13/359,336, a long range communication protocol defines both“regular” mode data units that are down-clocked by a value N₁ and“extended range” data units that are down-clocked by a value N₂, whereN₂>N₁. Thus, in some embodiments, a device (e.g., the AP 14 and/or aclient station 25) selectively transmits a long range data unit at afirst down-clocked rate or at a second, further down-clocked rate,depending on whether the device is in regular mode or extended rangemode.

Due to the use of multiple clock rates to generate coexisting data units(e.g., short range and long range data units, and/or regular long rangeand extended long range data units) in a particular region, it ishelpful if a communication device (e.g., the AP 14 and/or a clientstation 25) receiving a data unit can determine or auto-detect the clockrate used to generate the data unit. As described in embodiments below,a first preamble portion of a data unit permits a receiving device touse a corresponding technique to auto-detect the clock rate of the dataunit (e.g., to auto-detect the clock rate of the OFDM-modulated portionof the data unit that follows the first preamble portion). In thesubsequently described embodiments, the OFDM-modulated portion of thedata unit for which a clock rate is being detected is referred to as the“OFDM portion”. In some embodiments (e.g., in some embodiments where thefirst preamble portion includes an STF), however, the first preambleportion is also OFDM-modulated.

In a first group of example embodiments, comprising a first, second, andthird example embodiment and corresponding to FIGS. 7-9, an STF of thepreamble of a data unit is designed to indicate the clock rate of theOFDM portion of the data unit. The STF is used for one or more of packetdetection, initial synchronization, automatic gain control, etc., invarious embodiments. The OFDM portion includes one or more LTFs, whichin various embodiments are used for one or more of channel estimation,fine synchronization, etc. The preamble designs of FIGS. 7-9 areutilized in data units transmitted and/or received over a communicationchannel by a communication device (e.g., the AP 14 and/or a clientstation 25 of FIG. 1), in various embodiments or scenarios. Each ofFIGS. 7-9 illustrates two example preambles, each reflecting a PHY modethat corresponds to a particular clock rate. In one embodiment, an AP(e.g., AP 14) is capable of generating both example preambles (i.e., theAP supports multiple PHY modes corresponding to different clock rates),while each client station (e.g., each of client stations 25) is onlycapable of generating one of the example preambles (i.e., each clientstation only supports a PHY mode corresponding to a single clock rate).In another embodiment, both the AP and one or more of the clientstations are capable of generating both example preambles.

For ease of explanation, FIGS. 7-9 show preambles that include only afirst portion having a single STF and a second portion having a singleLTF. In other embodiments, however, different types and/or numbers offields are included in the preamble. For example, in an embodiment, thepreamble includes multiple LTFs following the STF. As another example,in an embodiment, additional, non-LTF fields of the preamble (e.g., oneor more SIG fields used for signaling basic PHY parameters to thereceiver) follow the LTF. In some embodiments, the preambles are thesame as any one of the preambles discussed above in connection withFIGS. 2-5, but with the first STF being designed as in one of theembodiments described below in connection with FIGS. 7-9. For example,in various embodiments, L-STF 102 of FIG. 2, L-STF 122 of FIG. 3,HT-GF-STF 142 of FIG. 4, or L-STF 172 of FIG. 5 is designed according toan embodiment described below. Moreover, while FIGS. 7-9 each showpreambles corresponding to only two possible clock rates, one ofordinary skill in the art will understand that the preamble designs andauto-detection techniques described below can be extended to systemsincluding three or more coexisting PHY modes with different clock rates.

In the first example embodiment (discussed with respect to FIG. 7), anSTF of the preamble is down-clocked using the same down-clocking ratioas a subsequent OFDM portion of the data unit. Referring to FIG. 7, afirst preamble 300 is included in data units that have an OFDM portionclocked at a first clock rate (e.g., the normal clock rate of an IEEE802.11a, 802.11n, or 802.11ac data unit, in an embodiment). The preamble300 includes a first preamble portion 310 and a second preamble portion314. The first preamble portion 310 is clocked at a first clock rate andincludes J repeating STF sequences 318-1 through 318-J. The secondpreamble portion 314 includes at least a first long training field(LTF1) 324, and is included in the OFDM portion of the data unit. Insome embodiments, the OFDM portion of the data unit also includes a dataportion (not shown in FIG. 7).

A second preamble 330 is included in data units that have an OFDMportion clocked at a second clock rate lower than the first clock rate.In the example embodiment of FIG. 7, the OFDM portion of a data unitwith preamble 330 is clocked at a rate equal to ¼ the clock rate of theOFDM portion of the data unit with preamble 300 (e.g., generated bydown-clocking from the first clock rate using N=4, in an embodiment). Inother embodiments, the second clock rate differs from the first clockrate by a different ratio (e.g., a down-clocking ratio of N=8, 10, 16,etc. is used, in various embodiments). Similar to the preamble 300, thepreamble 330 includes a first preamble portion 340 and a second preambleportion 344, with the first preamble portion 340 including J repeatingSTF sequences 348-1 through 348-J. Also similar to the preamble 300, thesecond preamble portion 344 includes at least a first long trainingfield (LTF1) 354, and is included in the OFDM portion of the data unit.Unlike the first preamble portion 310 of preamble 300, however, thefirst preamble portion 340 of preamble 330 is clocked at the lower,second clock rate.

Because the STF sequences 348 are generated using a clock rate fourtimes slower than the clock rate used to generate STF sequences 318, andbecause the first preamble portion 310 and the first preamble portion340 include the same number (J) of repeating STF sequences, the firstpreamble portion 340 of preamble 330 is four times longer in durationthan the first preamble portion 310 of preamble 300. A communicationdevice receiving data units having preamble 300 and data units havingpreamble 330 can therefore take advantage of the length between thestart of the first preamble portion and the end of the first preambleportion (i.e., in the embodiment of FIG. 7, between the start of thefirst preamble portion and the STF/LTF boundary) to determine the clockrate of the OFDM portion before demodulating OFDM symbols within theOFDM portion. To this end, and as shown in FIG. 7, a receiver performsautocorrelations on each received data unit. In one embodiment, a firstautocorrelation is performed using a repetition period (time interval)corresponding to a first potential clock rate, and a secondautocorrelation is performed using a repetition period corresponding toa second potential clock rate. In the example embodiment of FIG. 7, thefirst autocorrelation utilizes a 0.8 μs interval corresponding to the0.8 μs length of the STF sequences 318 clocked at the first clock rate,and the second autocorrelation utilizes a 3.2 μs interval correspondingto the 3.2 μs length of the STF sequences 348 clocked at the secondclock rate. The first and the second autocorrelation are simultaneouslyperformed by parallel carrier sense circuits and/or software modules ofa PHY unit such as PHY unit 20 or PHY unit 29 of FIG. 1, in anembodiment.

As shown in FIG. 7, the first autocorrelation outputs a first carriersense signal 380, and the second autocorrelation outputs a secondcarrier sense signal 384. In some embodiments, the pulse length of thefirst carrier sense signal 380 corresponds to an estimation of a lengthof time between sensing a carrier (depicted as the event CS 386) anddetecting a transition from the first preamble portion 310 to the secondpreamble portion 314 (depicted as the event “STF/LTF Boundary” 388).Similarly, in some embodiments, the pulse length of the second carriersense signal 384 corresponds to an estimation of a length of timebetween sensing a carrier (depicted as the event CS 390) and detecting atransition from the first preamble portion 340 to the second preambleportion 344 (depicted as the event “STF/LTF Boundary” 392).

In some embodiments, the carrier sense signal 380 and/or the carriersense signal 384 is/are compared to a suitable predetermined threshold,and is/are determined to be “high” when meeting the threshold. In someembodiments, detecting CS 386 or CS 390 includes determining that such athreshold is met. Moreover, in some embodiments, a transition from thefirst preamble portion to the second preamble portion is detected whenan autocorrelation goes below such a threshold (or below a different,second threshold) after having been “high” for a time period. In someembodiments, detecting STF/LTF Boundary 388 or STF/LTF Boundary 392includes detecting such a transition. Although FIG. 7 represents thefirst carrier sense signal 380 and the second carrier sense signal 384as continuous pulses, the term “pulse” as used herein includes bothcontinuous and non-continuous pulses (e.g., signals that are notnecessarily “high” or “low” for the entire pulse length, but meet somesuitable predetermined criteria for the entire pulse length).

In some embodiments, a receiver detects the clock rate of a receiveddata unit by determining which carrier sense signal indicates a strongautocorrelation when operating on the STF portion. For example, if thesecond carrier sense signal 384 rises but the first carrier sense signal380 does not rise, the receiver determines that the received STFsequences (and therefore, the corresponding OFDM portion of the dataunit) are clocked at the lower, second clock rate, in an embodiment.Conversely, in this embodiment, if the first carrier sense signal 380rises but the second carrier sense signal 384 does not rise, thereceiver determines that the received STF sequences (and therefore, thecorresponding OFDM portion of the data unit) are clocked at the higher,first clock rate. In other words, as an example, if the second carriersense signal 384 meets suitable detection criteria but the first carriersense signal 380 does not meet suitable detection criteria, the receiverdetermines that the received STF sequences (and therefore, thecorresponding OFDM portion of the data unit) are clocked at the lower,second clock rate, in an embodiment. Conversely, in this embodiment, ifthe first carrier sense signal 380 meets suitable detection criteria butthe second carrier sense signal 384 does not meet suitable detectioncriteria, the receiver determines that the received STF sequences (andtherefore, the corresponding OFDM portion of the data unit) are clockedat the higher, first clock rate.

In some instances, however, an STF sequence clocked at a higher rate cantrigger carrier sensing corresponding to a lower clocked rate. Forexample, a received data unit with STF sequences 318 (clocked at thehigher, first clocked rate) can cause both the first carrier sensesignal 380 and the second carrier sense signal 384 to indicate detectionof a carrier, in some embodiments and/or scenarios. In this situation,the receiver determines the clock rate of the OFDM portion of the dataunit based on a pulse length of at least one of the carrier sensesignals 380, 384, in an embodiment. For example, in one embodiment whereeach STF includes J=10 sequences (e.g., the first preamble portion 310of FIG. 7 is 8.0 μs long and the second preamble portion 340 of FIG. 7is 32 μs long), the receiver determines that the clock rate of the OFDMportion is the first clock rate when the carrier sense signals 380, 384indicate an 8.0 μs length between a start of carrier detection and theSTF/LTF boundary, and determines that the clock rate of the OFDM portionis the second clock rate when the carrier sense signals 380, 384indicate a 32 μs length between a start of carrier detection and theSTF/LTF boundary. Various other embodiments use other algorithms. As oneexample, where J=10, a receiver determines that the clock rate is thefirst clock rate when the STF/LTF boundary occurs within 10 μs of thestart of carrier detection, and determines that the clock rate is thesecond clock rate when the STF/LTF boundary occurs more than 10 μs afterthe start of carrier detection, in an embodiment. In some embodimentsand scenarios where both the carrier sense signal 380 and the carriersense signal 384 indicate detection of a carrier, the receiverdetermines the clock rate by observing the pulse length of only one ofthe carrier sense signals. In other embodiments, the respective pulselengths of both carrier sense signals are observed.

Because both carrier sense signals 380, 384 may initially indicate acarrier sense, a receiver using the example auto-detection technique ofFIG. 7 may not be able to determine the clock rate of a received dataunit until a time well after the beginning of the first preambleportion, and therefore may not have enough time to dynamically adjustthe receiver clock rate based on the detected clock rate. Accordingly, areceiver employing the auto-detection technique of FIG. 7 operates at aclock rate corresponding to the faster of the first and the second clockrates, in an embodiment.

In the second example embodiment (discussed with respect to FIG. 8), anSTF of the preamble includes repeated STF sequences that are generatedusing a constant clock rate, regardless of the clock rate of thesubsequent OFDM portion of the data unit. The STF, however, provides anindication of the clock rate of the OFDM portion by including more orfewer repetitions of the STF sequence. Referring to FIG. 8, a firstpreamble 400 is included in data units that have an OFDM portion clockedat a first clock rate (e.g., the normal clock rate of an IEEE 802.11a,802.11n, or 802.11ac data unit). The preamble 400 includes a firstpreamble portion 410 and a second preamble portion 414. The firstpreamble portion 410 includes J repeating STF sequences 418-1 through418-J. The second preamble portion 414 includes at least a first longtraining field (LTF1) 424, and is included in the OFDM portion of thedata unit. In some embodiments, the OFDM portion of the data unit alsoincludes a data portion (not shown in FIG. 8).

A second preamble 430 is included in data units that have an OFDMportion clocked at a second clock rate lower than the first clock rate.For example, in an embodiment, the OFDM portion of a data unit withpreamble 430 is clocked at a rate equal to ¼ the clock rate of the OFDMportion of the data unit with preamble 400 (e.g., generated bydown-clocking from the first clock rate using N=4, in an embodiment). Inother embodiments, the second clock rate differs from the first clockrate by a different ratio (e.g., a down-clocking ratio of N=8, 10, 16,etc. is used, in various embodiments). Similar to the preamble 400, thepreamble 430 includes a first preamble portion 440 and a second preambleportion 444, with the second preamble portion 444 including at least afirst long training field (LTF1) 454 and being included in the OFDMportion of the data unit. Moreover, in an embodiment, the first preambleportion 440 of preamble 430 is clocked at the same clock rate as thefirst preamble portion 410 of preamble 400 (e.g., both being clocked atthe first clock rate, or both being clocked at the second clock rate,according to various embodiments). Unlike the first preamble portion410, however, the first preamble portion 440 includes K repeating STFsequences 448-1 through 448-K, where K is greater than J. In someembodiments, the ratio K/J is equal to the ratio of the first clock rateto the second clock rate. For example, in one embodiment where the firstclock rate is four times greater than the second clock rate, the ratioK/J=4. In other embodiments, the ratio K/J is different than the ratioof the first clock rate to the second clock rate.

Because the first preamble portion 440 of preamble 430 includes more STFsequences than the first preamble portion 410 of preamble 400, the firstpreamble portion 440 is longer than the first preamble portion 410. Acommunication device receiving data units having preamble 400 and dataunits having preamble 430 can therefore take advantage of the lengthbetween the start of the first preamble portion and the end of the firstpreamble portion (i.e., in the embodiment of FIG. 8, between the startof the first preamble portion and the STF/LTF boundary) to determine theclock rate of the OFDM portion before demodulating OFDM symbols withinthe OFDM portion. To this end, and as shown in FIG. 8, a receiverperforms an autocorrelation on each received data unit. Unlike the firstexample embodiment of FIG. 7, only one autocorrelation is performed on areceived data unit, in an embodiment. In one embodiment, theautocorrelation is performed using a repetition period (time interval)corresponding to the clock rate used to generate both first preambleportion 410 and first preamble portion 440. In the example embodiment ofFIG. 8, the autocorrelation utilizes a 3.2 μs interval corresponding tothe 3.2 μs length of the STF sequences 418 and the STF sequences 448.

Whereas FIG. 7 illustrates alternative autocorrelation outputs thatcorrespond to outputs of different (e.g., parallel) carrier sensecircuits and/or software modules, both autocorrelation outputs shown inFIG. 8 represent alternative outputs of the same carrier sense circuitand/or software module. A first carrier sense signal 480 is output bythe autocorrelation when a data unit with preamble 400 is received, anda second carrier sense signal 484 is output by the autocorrelation whena data unit with preamble 430 is received. In some embodiments, thepulse length of the first carrier sense signal 480 corresponds to anestimation of a length of time between sensing a carrier (depicted asthe event CS 486) and detecting a transition from the first preambleportion 410 to the second preamble portion 414 (depicted as the event“STF/LTF Boundary” 488). Similarly, in some embodiments, the pulselength of the second carrier sense signal 484 corresponds to anestimation of a length of time between sensing a carrier (depicted asthe event CS 490) and detecting a transition from the first preambleportion 440 to the second preamble portion 444 (depicted as the event“STF/LTF Boundary” 492).

In some embodiments, a receiver determines the clock rate of the OFDMportion of a received data unit based on a pulse length of the carriersense signal. For example, in one embodiment where J=4 and K=16 (e.g.,the first preamble portion 410 in the example embodiment of FIG. 8 is12.8 μs long and the second preamble portion 440 of FIG. 8 is 51.2 μslong), the receiver determines that the clock rate of the OFDM portionis the first clock rate when the carrier sense signal indicates a 12.8μs length between a carrier sense and the STF/LTF boundary, anddetermines that the clock rate of the OFDM portion is the second clockrate when the carrier sense signal indicates a 51.2 μs length between acarrier sense and the STF/LTF boundary. Various other embodiments useother algorithms. As one example, again where J=4 and K=16, a receiverdetermines that the clock rate is the first clock rate when the STF/LTFboundary occurs within 20 μs of the carrier sense, and determines thatthe clock rate is the second clock rate when the STF/LTF boundary occursmore than 20 μs after the carrier sense, in an embodiment. In otherembodiments, different suitable values of J and K are utilized.

As with the example auto-detection technique of FIG. 7, the exampleauto-detection technique of FIG. 8 may not provide enough time todynamically adjust the receiver clock rate based on the detected clockrate. Accordingly, a receiver employing the auto-detection technique ofFIG. 8 operates at a clock rate corresponding to the faster of the firstand the second clock rates, in an embodiment.

In the third example embodiment (discussed with respect to FIG. 9), anSTF of the preamble includes repeated STF sequences that are generatedusing a constant clock rate, regardless of the clock rate of thesubsequent OFDM portion of the data unit. A repeated STP sequence,however, is augmented by a cover code that provides an indication of theclock rate of the OFDM portion. In one embodiment, this preamble designis combined with the preamble design of FIG. 8, where the clock rate ofthe OFDM portion is additionally indicated by the number of repetitionsof the STF sequence. FIG. 9 illustrates an example preamble design andcorresponding auto-detection technique for an embodiment where both acover code and the number of STF sequences are used to indicate theclock rate of the OFDM portion. Referring to FIG. 9, a first preamble500 is included in data units that have an OFDM portion clocked at afirst clock rate (e.g., the normal clock rate of an IEEE 802.11a,802.11n, or 802.11ac data unit, in various embodiments). The preamble500 includes a first preamble portion 510 and a second preamble portion514. The first preamble portion 510 includes J repeating STF sequences518-1 through 518-J that are augmented by a first cover code. The firstcover code corresponds to the first clock rate (i.e., is used toindicate to a receiving device that the first clock rate is used for theOFDM portion of the data unit). The second preamble portion 514 includesat least a first long training field (LTF1) 524, and is included in theOFDM portion of the data unit. In some embodiments, the OFDM portion ofthe data unit also includes a data portion (not shown in FIG. 9).

A second preamble 530 is included in data units that have an OFDMportion clocked at a second clock rate lower than the first clock rate.For example, in an embodiment, the OFDM portion of a data unit withpreamble 530 is clocked at a rate equal to ¼ the clock rate of the OFDMportion of the data unit with preamble 500 (e.g., by down-clocking fromthe first clock rate using N=4, in an embodiment). In other embodiments,the second clock rate differs from the first clock rate by a differentratio (e.g., a down-clocking ratio of N=8, 10, 16, etc. is used, invarious embodiments). Similar to the preamble 500, the preamble 530includes a first preamble portion 540 and a second preamble portion 544,with the second preamble portion 544 including at least a first longtraining field (LTF1) 554 and being included in the OFDM portion of thedata unit. Moreover, in an embodiment, the first preamble portion 540 ofpreamble 530 is clocked at the same clock rate as the first preambleportion 510 of preamble 500 (e.g., both being clocked at the first clockrate, or both being clocked at the second clock rate, according tovarious embodiments). Unlike the first preamble portion 510, however,the first preamble portion 540 includes STF sequences 548 that areaugmented by a second cover code different than the first cover code.The second cover code corresponds to the lower, second clock rate (i.e.,is used to indicate to a receiving device that the OFDM portion of thedata unit is clocked at the second clock rate). In an embodiment, thefirst cover code, used in preamble 500, is a series of only positiveones (i.e., [1 1 1 1 . . . ]), while the second cover code, used inpreamble 530, is a series of alternating positive and negative ones(i.e., [1 −1 1 −1 . . . ]).

As seen in FIG. 9, the first preamble portion 540 of preamble 530includes K repeating STF sequences 548-1 through 548-K. In embodimentssuch as the example embodiment illustrated in FIG. 9, where the clockrate of the OFDM portion is additionally indicated by the number of STFsequences, K is greater than J. In some of these embodiments, the ratioK/J is equal to the ratio of the first clock rate to the second clockrate. Alternatively, in some embodiments where the clock rate of theOFDM portion is not indicated by the number of STF sequences, K is equalto J.

Because the cover code of the first preamble section is not known apriori for a particular received data unit, a receiver processes thefirst preamble portion of the received data unit in two parallel paths,in an embodiment. In a first path the receiver attempts to remove orundo the first cover code, and in a second path the receiver attempts toremove or undo the second cover code, in an embodiment. A firstautocorrelation of at least the first preamble portion (as processed)follows the cover code processing in the first path, and a second,parallel autocorrelation of at least the first preamble portion (asprocessed) follows the cover code processing in the second path, in anembodiment. For example, in one embodiment where the first cover code isa series of positive ones and the second cover code is a series ofalternating positive ones and negative ones, the first autocorrelationis a conventional autocorrelation, but the samples of one of the twowindows of the second autocorrelation are multiplied by negative one.

As shown in FIG. 9, the first autocorrelation outputs a first carriersense signal 580, and the second autocorrelation outputs a secondcarrier sense signal 584. In some embodiments, a receiver detects theclock rate of a received data unit by determining which carrier sensesignal indicates a strong autocorrelation when operating on the STFportion. For example, if the second carrier sense signal 584 rises(i.e., carrier sense 586 occurs) but the first carrier sense signal 580does not rise (i.e., carrier sense 590 does not occur), the receiverdetermines that the OFDM portion of the data unit is clocked at thelower, second clock rate, in an embodiment. Conversely, in thisembodiment, if the first carrier sense signal 580 rises but the secondcarrier sense signal 584 does not rise, the receiver determines that theOFDM portion of the data unit is clocked at the higher, first clockrate. In some embodiments where the clock rate is additionally indicatedby the number of STF sequences (i.e., where K>J), the receiver alsodetermines, or confirms, the clock rate based on the pulse length ofcarrier sense signal 580 and/or carrier sense signal 584 (e.g., theposition of the STF/LTF boundary 592 or the STF/LTF boundary 596),similar to the auto-detection method of FIG. 8. In other words, as anexample, if the second carrier sense signal 584 meets suitable detectioncriteria but the first carrier sense signal 580 does not meet suitabledetection criteria, the receiver determines that the corresponding OFDMportion of the data unit is clocked at the lower, second clock rate, inan embodiment. Conversely, in this embodiment, if the first carriersense signal 580 meets suitable detection criteria but the secondcarrier sense signal 584 does not meet suitable detection criteria, thereceiver determines that the OFDM portion of the data unit is clocked atthe higher, first clock rate.

A receiver using the example auto-detection technique of FIG. 9 isgenerally able to quickly determine the clock rate of a received dataunit (e.g., based on the carrier sense 586 or carrier sense 590). Thus,in an embodiment, the receiving device is configured to dynamicallyadjust the receiver clock rate to correspond to the determined clockrate of the OFDM portion, which can save power in the receiving device.In some of these embodiments, the receiving device is configured todynamically adjust the receiver clock rate in response to the occurrenceof carrier sense 586 or carrier sense 590, depending on whether a dataunit with an OFDM portion clocked at the first clock rate or the secondclock rate is received. In an embodiment, the receiving device isconfigured to dynamically adjust the receiver clock rate beforeprocessing (e.g., demodulating) any part of the OFDM portion of the dataunit.

In a second group of example embodiments, comprising fourth, fifth, andsixth example embodiments, and corresponding to FIGS. 10-12, an SC“extra preamble” portion serves as a first preamble portion of a longrange data unit, where the extra preamble portion is designed to reflectthe clock rate of the OFDM portion of the data unit. In someembodiments, formats of the data units of FIGS. 10-12 are similar to ashort range data unit, with the exception of the extra preamble portion.The extra preamble portion includes a SYNC field (e.g., similar to aSYNC field according to the IEEE 802.11b Standard) and, in someembodiments, an SFD field (e.g., similar to an SFD field according tothe IEEE 802.11b Standard). In these example embodiments, the SYNC fieldand/or the SFD field is/are designed to reflect the clock rate of theOFDM portion of the data unit. In some embodiments where a data unit isgenerated for transmission via multiple, aggregated 20 MHz channels(e.g., via a 40 MHz, 80 MHz, 160 MHz, etc. channel), the extra preambleportion is repeated in each 20 MHz sub-band.

The extra preamble portion is sampled or clocked at a lower rate thanthe OFDM portion of the data unit, in some embodiments. For example, inone embodiment, the extra preamble portion is down-clocked from the IEEE802.11b rate of 11 MHz by a down-clock ratio N equal to a down-clockingratio used for the OFDM portion of the data unit. As another example, inanother embodiment, the extra preamble portion is sampled or clockedwith approximately ⅔ the clock rate of the regular (not down-clocked)OFDM portion. In some embodiments where the extra preamble portion issampled or clocked at a different rate than the OFDM portion, one ormore particular requirements with respect to the SC/OFDM boundarybetween the extra preamble portion and the OFDM portion are satisfied.For example, the SC/OFDM boundary requirement defined in the IEEE802.11g Standard is satisfied, in an embodiment.

The preamble designs of FIGS. 10-12 are utilized in data unitstransmitted and/or received over a communication channel by acommunication device (e.g., the AP 14 and/or a client station 25 of FIG.1), in various embodiments or scenarios. Each of FIGS. 10-12 illustratestwo example preambles, each reflecting a PHY mode that corresponds to aparticular clock rate of the OFDM portion. In one embodiment, an AP(e.g., AP 14) is capable of generating both example preambles (i.e., theAP supports multiple PHY modes corresponding to different clock rates),while each client station (e.g., each of client stations 25) is onlycapable of generating one of the example preambles (i.e., each clientstation only supports a PHY mode corresponding to a single clock rate).In another embodiment, both the AP and one or more of the clientstations are capable of generating both example preambles.

In some embodiments, the preambles of FIGS. 10-12 include differenttypes and/or numbers of fields than shown. For example, in anembodiment, additional fields are included between the extra preambleportion and the STF (or, in embodiments without an STF, between theextra preamble portion and an LTF). In some embodiments, the preamblesare the same as any one of the preambles discussed above in connectionwith FIGS. 2-5, but with one of the extra preamble portions describedbelow in connection with FIGS. 10-12 being added at the beginning of thepreamble. For example, in various embodiments, the extra preambleportion is added before L-STF 102 of FIG. 2, L-STF 122 of FIG. 3,HT-GF-STF 142 of FIG. 4, or L-STF 172 of FIG. 5. Moreover, while FIGS.10-12 each show preambles corresponding to only two possible clockrates, one of ordinary skill in the art will understand that thepreamble designs and auto-detection techniques described below can beextended to systems including three or more coexisting PHY modes withdifferent clock rates.

In the fourth example embodiment (discussed with respect to FIG. 10),the preamble includes a SYNC field that is generated using a constantclock rate, regardless of the clock rate of the OFDM portion of thecorresponding data unit. The SYNC field, however, has a particularlength based on the clock rate of the OFDM portion, allowing a receiverto differentiate between clock rates based on the SYNC field. Referringto FIG. 10, a first preamble 600 is included in data units that have anOFDM portion clocked at a first clock rate (e.g., the normal clock rateof an IEEE 802.11a, 802.11n, or 802.11ac data unit, in variousembodiments). The preamble 600 includes a first preamble portion 610(i.e., the “extra preamble” portion) and a second preamble portion 612.The first preamble portion 610 includes a SYNC field 614 and an SFDfield 616. In an embodiment, the first preamble portion includes anumber of repetitions of a Barker code. The second preamble portion 612is included in the OFDM portion of the data unit, and includes an STF620 and a preamble portion 622 with one or more LTFs and a SIG field. Insome embodiments, the OFDM portion of the data unit also includes a dataportion (not shown in FIG. 10). In an alternative embodiment, thepreamble 600 does not include SFD field 616. In another alternativeembodiment, SFD field 616 is included in preamble 600 but STF 620 is notincluded.

A second preamble 630 is included in data units that have an OFDMportion clocked at a second clock rate lower than the first clock rate.For example, in an embodiment, the OFDM portion of a data unit withpreamble 630 is clocked at a rate equal to ¼ the clock rate of the OFDMportion of the data unit with preamble 600 (e.g., by down-clocking fromthe first clock rate using N=4, in an embodiment). In other embodiments,the second clock rate differs from the first clock rate by a differentsuitable ratio (e.g., a down-clocking ratio of N=8, 10, 16, etc. isused, in various embodiments). Similar to the preamble 600, the preamble630 includes a first preamble portion 640 and a second preamble portion642, with the first preamble portion 640 including a SYNC field 644 andan SFD field 646. Also similar to the preamble 600, the second preambleportion 642 is included in the OFDM portion, and includes an STF 650 anda preamble portion 652 with one or more LTFs and a SIG field. Further,in an embodiment, the first preamble portion 640 of preamble 630 isclocked at the same clock rate as the first preamble portion 610 ofpreamble 600 (e.g., both being clocked at the first clock rate, or bothbeing clocked at the second clock rate, according to variousembodiments). SYNC field 644, however, is longer than SYNC field 614 ofpreamble 600. In one embodiment, SYNC field 644 includes a number ofrepetitions of a Barker code that is greater than a number ofrepetitions of the same Barker code in SYNC field 614. For example, SYNCfield 644 includes a number of repetitions of a Barker code that is Ntimes greater than the number of repetitions of the same Barker code inSYNC field 614 when the first clock rate is N times greater than thesecond clock rate. In some embodiments, the SFD field is also utilizedto differentiate between the first and second clock rates. In theseembodiments, SFD field 646 of preamble 630 is different than SFD field616 of preamble 600.

In an embodiment, a communication device receiving data units havingpreamble 600 and data units having preamble 630 takes advantage of thedifferent SYNC fields (and, in some embodiments, the different SFDfields) of preamble 600 and preamble 630 to determine the clock rate ofthe OFDM portion before demodulating OFDM symbols in the OFDM portion.In some embodiments, the receiver performs an autocorrelation to detectwhich SYNC field (and therefore, which OFDM portion clock rate) is usedin the received packet.

In the fifth example embodiment (discussed with respect to FIG. 11), thepreamble again includes a SYNC field that is generated using a constantclock rate, regardless of the clock rate of the OFDM portion of thecorresponding data unit. The SYNC field, however, includes a particularrepeated sequence based on the clock rate of the OFDM portion, allowinga receiver to differentiate between clock rates based on the SYNC field.Referring to FIG. 11, a first preamble 700 is included in data unitsthat have an OFDM portion clocked at a first clock rate (e.g., thenormal clock rate of an IEEE 802.11a, 802.11n, or 802.11ac data unit, invarious embodiments). The preamble 700 includes a first preamble portion710 (i.e., the “extra preamble” portion) and a second preamble portion712. The first preamble portion 710 includes a SYNC field 714 and astart frame delimiter field SFD field 716. SYNC field 714 includes afirst repeated sequence (Ga). In an embodiment, the first repeatedsequence is a first Golay sequence. The second preamble portion 712 isincluded in the OFDM portion of the data unit, and includes an STF 720and a preamble portion 722 with one or more LTFs and a SIG field. Insome embodiments, the OFDM portion of the data unit also includes a dataportion (not shown in FIG. 11). In an alternative embodiment, thepreamble 700 does not include SFD field 716. In another alternativeembodiment, SFD field 716 is included in preamble 700, but STF 720 isnot included.

A second preamble 730 is included in data units that have an OFDMportion clocked at a second clock rate lower than the first clock rate.For example, in an embodiment, the OFDM portion of a data unit withpreamble 730 is clocked at a rate equal to ¼ the clock rate of the OFDMportion of the data unit with preamble 700 (e.g., by down-clocking fromthe first clock rate using N=4, in an embodiment). In other embodiments,the second clock rate differs from the first clock rate by a differentsuitable ratio (e.g., a down-clocking ratio of N=8, 10, 16, etc. isused, in various embodiments). Similar to the preamble 700, the preamble730 includes a first preamble portion 740 and a second preamble portion742, with the first preamble portion 740 including a SYNC field 744.Also similar to the preamble 700, the second preamble portion 742 isincluded in the OFDM portion, and includes an STF 750 and a preambleportion 752 with one or more LTFs and a SIG field. Further, in anembodiment, the first preamble portion 740 of preamble 730 is clocked atthe same clock rate as the first preamble portion 710 of preamble 700(e.g., both being clocked at the first clock rate, or both being clockedat the second clock rate, according to various embodiments). SYNC field744, however, includes a second repeated sequence (Gb) different thanthe first repeated sequence Ga. In some embodiments, the second repeatedsequence is a second Golay sequence complementary to the first Golaysequence. In some embodiments, the sequences Ga and Gb are suitablecomplementary sequences other than Golay sequences. In an embodiment,complementary sequences Ga and Gb are selected so that the sum ofcorresponding out-of-phase aperiodic autocorrelation coefficients of thesequences Ga and Gb is zero. In some embodiments, complementarysequences Ga and Gb have a zero or almost-zero periodiccross-correlation. In another aspect, sequences Ga and Gb have aperiodiccross-correlation with a narrow main lobe and low-level side lobes, oraperiodic auto-correlation with a narrow main lobe and low-level sidelobes.

Generally, the two complementary sequences of SYNC field 714 and SYNCfield 744 have correlation properties suitable for detection at areceiving device. In embodiments where the sequences are Golaysequences, Golay sequences of length 16, 32, 64, 128, or any othersuitable length are utilized for the complementary sequences. In anembodiment, pi/2 chip-level rotation is applied to the Golay codesequences in the same manner as defined in the IEEE802.11ad Standard.

The preamble 730 also includes a start frame delimiter field SFD field746, which in some embodiments is different than SFD field 716. In analternative embodiment, where preamble 700 does not include SFD field716, the preamble 730 does not include SFD field 746. In anotheralternative embodiment, where preamble 700 includes SFD field 716 butnot STF 720, preamble 730 includes SFD field 746 but not STF 750. In anembodiment, both SFD field 716 and SFD field 746 each include one ormore of the sequences (e.g., Golay sequences) repeated in the SYNCfield, but augmented by a sign flip (i.e., reversed polarity, e.g., −Gaor −Gb). In another embodiment, both SFD field 716 and SFD field 746include a sequence that is complementary to the repeated sequence of theSYNC field. For example, SFD field 746 of preamble 730 includes a firstGolay sequence utilized in SYNC field 714 of preamble 700, and SFD field716 of preamble 700 includes a second Golay sequence utilized in SYNCfield 744 of preamble 730, in an embodiment.

In an embodiment, a communication device receiving data units havingpreamble 700 and data units having preamble 730 takes advantage of thedifferent SYNC fields (and, in some embodiments, the different SFDfields) of preamble 700 and preamble 730 to determine the clock rate ofthe OFDM portion before demodulating OFDM symbols in the OFDM portion.In some embodiments, the receiver performs parallel cross-correlations,each of which correlates the received sequences with one of the possibleSYNC field sequences, and compares the outputs of the cross-correlationsto determine which SYNC field (and, therefore, which OFDM portion clockrate) is used in the received packet.

In the sixth example embodiment (discussed with respect to FIG. 12), thepreamble includes a SYNC field that does not change based on the OFDMportion clock rate. However, the preamble includes a different SFD fieldfor each different clock rate. Referring to FIG. 12, a first preamble800 is included in data units that have an OFDM portion clocked at afirst clock rate (e.g., the nolinal clock rate of an IEEE 802.11a,802.11n, or 802.11ac data unit, in various embodiments). The preamble800 includes a first preamble portion 810 (i.e., the “extra preamble”portion) and a second preamble portion 812. The first preamble portion810 includes a SYNC field 814 and a first start frame delimiter (SFD1)field 816. The second preamble portion 812 is included in the OFDMportion of the data unit, and includes an STF 820 and a preamble portion822 with one or more LTFs and a SIG field. In some embodiments, the OFDMportion of the data unit also includes a data portion (not shown in FIG.12). In an alternative embodiment, the preamble 800 does not include STF820.

A second preamble 830 is included in data units that have an OFDMportion clocked at a second clock rate lower than the first clock rate.For example, in an embodiment, the OFDM portion of a data unit withpreamble 830 is clocked at a rate equal to ¼ the clock rate of the OFDMportion of the data unit with preamble 800 (e.g., by down-clocking fromthe first clock rate using N=4, in an embodiment). In other embodiments,the second clock rate differs from the first clock rate by a differentsuitable ratio (e.g., a down-clocking ratio of N=8, 10, 16, etc. isused, in various embodiments). Similar to the preamble 800, the preamble830 includes a first preamble portion 840 and a second preamble portion842, with the first preamble portion 840 including a SYNC field 844.Also similar to the preamble 800, the second preamble portion 842 isincluded in the OFDM portion, and includes an STF 850 and a preambleportion 852 with one or more LTFs and a SIG field. Further, in anembodiment, the first preamble portion 840 of preamble 830 is clocked atthe same clock rate as the first preamble portion 810 of preamble 800(e.g., both being clocked at the first clock rate, or both being clockedat the second clock rate, according to various embodiments). The firstpreamble portion 840, however, includes a second start frame delimiter(SFD2) field 846 that is different than SFD1 field 816. For example, inone embodiment, SFD1 field 816 includes one or more sequences that arerepeated in SYNC field 814 but with a sign flip, while SFD2 field 846includes the one or more sequences without the sign flip. As anotherexample, in one embodiment, SFD1 field 816 includes one or morerepetitions of a sequence that is different than a sequence repeated oneor more times in SFD2 field 846.

In an embodiment, a communication device receiving data units havingpreamble 800 and data units having preamble 830 takes advantage of thedifferent SFD fields of preamble 800 and preamble 830 to determine theclock rate of the OFDM portion before demodulating OFDM symbols in theOFDM portion. In some embodiments where SFD1 field 816 and SFD2 field846 include different sequences, or where SFD2 field 846 includes thesame sequence as SFD1 field 816 but with a sign flip, a receiverperforms parallel cross-correlations to detect which SFD (and,therefore, which OFDM portion) is used in the received packet.

FIG. 13 is a flow diagram of an example method 900 for generating a dataunit according to the first, second, third, fourth, fifth, or sixthexample preamble design (of which example embodiments are shown in FIGS.7-12, respectively), according to an embodiment. In some embodiments, anAP such as the AP 14 of FIG. 1 (and/or a client station such as theclient station 25-1) is configured to implement the method 900 togenerate a data unit for transmission over a communication channel.

At block 902, a first preamble portion is generated based on a PHY mode.More specifically, the first preamble portion is generated based atleast on whether the PHY mode is a first PHY mode or a second PHY mode.In an embodiment, the first and second PHY modes correspond toparticular communication protocols or particular modes of acommunication protocol. For example, in one embodiment, the first PHYmode corresponds to a short range communication protocol and the secondPHY mode corresponds to a long range communication protocol. As anotherexample, in an embodiment, the first PHY mode corresponds to a regularmode of a long range communication protocol and the second PHY modecorresponds to an extended range mode of the long range communicationprotocol. In some embodiments, the first preamble portion is generatedalso based on whether the PHY mode is one or more other possible PHYmodes (e.g., a third PHY mode, a fourth PHY mode, etc.). For example, inan embodiment, the first PHY mode corresponds to a short rangecommunication protocol, the second PHY mode corresponds to a regularmode of a long range communication protocol, and a third PHY modecorresponds to an extended range mode of the long range communicationprotocol.

In some embodiments, the first preamble portion is OFDM-modulated. Forexample, in an embodiment, the first preamble portion includes anOFDM-modulated STF. In other embodiments, the first preamble portionuses SC modulation. For example, in an embodiment, the first preambleportion includes an SC SYNC field. More specific examples of firstpreamble portions that are based on the PHY mode are described below inconnection with FIGS. 15, 17, 19, 21, and 23.

At block 904, an OFDM portion is generated using a first clock rate or asecond clock rate based on the PHY mode. More specifically, the OFDMportion is clocked at the first clock rate when the PHY mode is thefirst PHY mode, and is clocked at the second clock rate when the PHYmode is the second PHY mode. The second clock rate is lower than thefirst clock rate (e.g., in some embodiments, by an integer factor N).The OFDM portion follows the first preamble portion in the data unitbeing generated, and includes a second preamble portion that includesone or more LTFs. In some embodiments, the OFDM portion also includes adata portion of the data unit. In some embodiments, the OFDM portion isthe same as a corresponding portion of a short range data unit or longrange data unit as described in connection with FIGS. 2-5. In some ofthese embodiments, the design of the OFDM portion is based on the PHYmode.

Although FIG. 13 shows only blocks 902 and 904 in the method 900, someembodiments include additional method elements. For example, in anembodiment, a third method element after block 904 includes transmittingvia a communication channel (e.g., a wireless communication channel) adata unit that includes the generated first preamble portion and thegenerated OFDM portion. Moreover, although block 904 is shown later inthe flow diagram of example method 900 than block 902, block 904 occursbefore, or simultaneously with, block 902 in other embodiments.

FIG. 14 is a flow diagram of an example method 910 for auto-detecting aclock rate of a data unit generated according to the first, second,third, fourth, fifth, or sixth example preamble design (of which exampleembodiments are shown in FIGS. 7-12, respectively), according to anembodiment. In some embodiments, an AP such as the AP 14 of FIG. 1(and/or a client station such as the client station 25-1) is configuredto implement the method 910.

At block 912, a data unit is received via a communication channel. In anembodiment where the method 910 is implemented by an AP such as AP 14 ofFIG. 1, the data unit is received via an antenna such as one or more ofantennas 24 of FIG. 1 and a PHY unit such as PHY unit 20 of FIG. 1. Inan embodiment where the method 910 is implemented by a client stationsuch as client station 25-1 of FIG. 1, the data unit is received via anantenna such as one or more of antennas 34 of FIG. 1 and a PHY unit suchas PHY unit 29 of FIG. 1. In an embodiment, the communication channel isa wireless communication channel.

The data unit received at block 912 includes a first preamble portion,and an OFDM portion that follows the first preamble portion. The OFDMportion of the data unit includes a second preamble portion includingone or more LTFs. According to various embodiments, the received dataunit is a data unit having the preamble design described in connectionwith any one of FIGS. 7-12. Additionally or alternatively, according tovarious embodiments, the received data unit is a data unit generatedaccording to the method of any one of FIG. 15, 17, 19, 21, or 23,described below.

At block 914, a clock rate of the OFDM portion of the data unit receivedat block 912 is auto-detected or determined based on the first preambleportion of the data unit. More specifically, it is determined whetherthe clock rate is a first clock rate corresponding to a first PHY modeor a lower, second clock rate corresponding to a second PHY mode, in anembodiment. In various embodiments, the PHY modes are similar to any ofthe PHY modes described in connection with block 902 of method 900 inFIG. 13. More specific examples of how the clock rate of the OFDMportion is determined are described below in connection with FIGS. 16,18, 20, 22, and 24.

Although FIG. 14 shows only blocks 912 and 914 in the method 910, someembodiments include additional method elements. Moreover, while themethod 910 has been described with reference to determining a first or asecond clock rate, some embodiments additionally determine (at block914) whether the clock rate is a third clock rate, a third or a fourthclock rate, etc.

FIG. 15 is a flow diagram of an example method 920 for generating a dataunit according to the first example preamble design (of which an exampleembodiment is shown in FIG. 7), according to an embodiment. In someembodiments, an AP such as the AP 14 of FIG. 1 (and/or a client stationsuch as the client station 25-1) is configured to implement the method920 to generate a data unit for transmission over a communicationchannel.

At block 922, it is determined whether a PHY mode of a communicationdevice implementing the method 920 is a first PHY mode or a second PHYmode. In an embodiment, the first and second PHY modes correspond toparticular communication protocols or particular modes of acommunication protocol, such as described above in connection with block902 of method 900 in FIG. 13.

If it is determined at block 922 that the PHY mode is the first PHYmode, the flow proceeds to block 924. At block 924, a first preambleportion is generated using a first clock rate that corresponds to thefirst PHY mode. In various embodiments, the first preamble portion isOFDM-modulated (e.g., includes an OFDM-modulated STF, in an embodiment)or uses SC modulation (e.g., includes an SC SYNC field, in anembodiment).

At block 926, an OFDM portion is generated using the first clock rate.The OFDM portion follows the first preamble portion in the data unitbeing generated, and includes a second preamble portion that includesone or more LTFs. In an embodiment, the OFDM portion also includes adata portion of a data unit. In some embodiments, the OFDM portion isthe same as a corresponding portion of a short range data unit or longrange data unit as described in connection with FIGS. 2-5. In some ofthese embodiments, the design of the OFDM portion is based on the PHYmode determined at block 922.

On the other hand, if it is determined at block 922 that the PHY mode isthe second PHY mode, the flow proceeds to block 930. At block 930, afirst preamble portion is generated using a second clock rate thatcorresponds to the second PHY mode. In some embodiments, the firstpreamble portion generated at block 930 is the same as or similar to thefirst preamble portion generated at block 924, with the exception of theclock rate (and therefore length) of the first preamble portion. Forexample, in an embodiment, the first preamble portion generated at block930 uses the same type of modulation (e.g., OFDM, SC, etc.), andincludes the same repeating sequence and the same number of repetitionsof the sequence, as the first preamble portion generated at block 924.The second clock rate is lower than the first clock rate (e.g., in someembodiments, by an integer factor N).

At block 932, an OFDM portion is generated using the second clock rate.The OFDM portion follows the first preamble portion in the data unitbeing generated, and includes a second preamble portion that includesone or more LTFs. In some embodiments, the OFDM portion generated atblock 932 is the same as the OFDM portion generated at block 926, withthe exception of the clock rate (and therefore length) of the OFDMportion.

In some embodiments, the method 920 of FIG. 15 includes additionalmethod elements not shown. For example, in an embodiment, after block926 and after block 932, an additional method element includestransmitting via a communication channel (e.g., a wireless communicationchannel) a data unit that includes both the generated first preambleportion and the generated OFDM portion. Moreover, although blocks 926and 932 are shown later in the flow diagram of example method 920 thanblocks 924 and 930, respectively, blocks 926 and 932 occur before, orsimultaneously with, blocks 924 and 930 in other embodiments.

FIG. 16 is a flow diagram of an example method 940 for auto-detecting aclock rate of a data unit generated according to the first examplepreamble design (of which an example embodiment is shown in FIG. 7),according to an embodiment. In some embodiments, an AP such as the AP 14of FIG. 1 (and/or a client station such as the client station 25-1) isconfigured to implement the method 940.

At block 942, a data unit is received via a communication channel. Block942 is similar to block 912 of the method 910 in FIG. 14, in someembodiments. The data unit received at block 942 includes a firstpreamble portion, and an OFDM portion that follows the first preambleportion. The OFDM portion of the data unit includes a second preambleportion including one or more LTFs. According to various embodiments,the received data unit is a data unit having the preamble designdescribed in connection with FIG. 7. Additionally or alternatively,according to various embodiments, the received data unit is a data unitgenerated according to the method 900 of FIG. 15.

At block 944, a first autocorrelation of at least the first preambleportion of the data unit received at block 942 is performed, where thefirst autocorrelation is performed using a first repetition period andoutputs a first carrier sense signal. In an embodiment, the firstrepetition period is the same as a first potential length of a repeatingsequence in the first preamble portion. For example, for the examplepreamble design shown in FIG. 7, the first repetition period is equal to0.8 μs or another suitable duration.

At block 948, a second autocorrelation of at least the first preambleportion of the data unit received at block 942 is performed, where thesecond autocorrelation is performed using a second repetition period andoutputs a second carrier sense signal In an embodiment, the secondrepetition period is the same as a second potential length of arepeating sequence in the first preamble portion, different than thefirst potential length. For example, for the example preamble designshown in FIG. 7, the second repetition period is equal to 3.2 μs oranother suitable duration. In an embodiment, the first autocorrelationof block 944 is performed at least partially in parallel with the secondautocorrelation at block 948.

At block 950, it is determined whether both the first autocorrelationand the second autocorrelation indicate the presence of a carrier (e.g.,whether the first carrier sense signal and the second carrier sensesignal indicate the presence of a carrier, in an embodiment). Forexample, in an embodiment, it is determined whether both the firstcarrier sense signal and the second carrier sense signal are at a “high”level (or any other indicator of a relatively strong autocorrelation).

If it is determined at block 950 that both the first autocorrelation andthe second autocorrelation indicate the presence of a carrier, flowproceeds to block 952. At block 952, the clock rate of the OFDM portionof the data unit received at block 942 is determined based on a pulselength of the first carrier sense signal (i.e., the firstautocorrelation output,) and/or a pulse length of the second carriersense signal (i.e., the second autocorrelation output). In anembodiment, the clock rate of the OFDM portion is determined to be afirst clock rate if the first carrier sense signal and/or the secondcarrier sense signal are at a “high” level (or any other suitableindicator of a relatively strong autocorrelation) for a first length oftime (e.g., 0.8 μs), and is determined to be a lower, second clock rateif the first carrier sense signal and/or the second carrier sense signalare at a “high” level (or any other suitable indicator of a relativelystrong autocorrelation) for a longer, second length of time (e.g., 3.2μs). In some embodiments, the determination at block 950 is performed bydetermining whether a pulse length of the first and/or second carriersense signal is in a first length range (e.g., less than 10 μs) or asecond length range (e.g., greater than 10 μs). In an embodiment, thepulse length of the first and second carrier sense signals correspondsto the length of time between i) sensing a carrier and ii) detecting atransition from the first preamble portion of the received data unit tothe second preamble portion of the received data unit.

On the other hand, if it is determined at block 950 that the firstautocorrelation output or the second autocorrelation output (but notboth) does not indicate the presence of a carrier, flow proceeds toblock 954. At block 954, the clock rate of the OFDM portion of the dataunit received at block 942 is determined based on which autocorrelationindicates the presence of a carrier. For example, in an embodiment, theclock rate of the OFDM portion is determined to be a first clock ratewhen the first (but not the second) autocorrelation indicates a carriersense, and is determined to be a lower, second clock rate when thesecond (but not the first) autocorrelation indicates a carrier sense.

In some embodiments, the method 940 includes additional method elementsnot shown in FIG. 16. For example, in an embodiment, the method 940includes providing, prior to receiving the data unit at block 942, areceiver clock rate that corresponds to the first, higher potentialclock rate of received data units. Moreover, while the method 940 hasbeen described with reference to determining a first and a second clockrate, some embodiments include additional method elements (e.g., similarto blocks 944 and 948) corresponding to a third clock rate, a third anda fourth clock rate, etc., where it is also determined at block 952 or954 whether the clock rate of the OFDM portion is one of theseadditional potential clock rates. In embodiments utilizing a third ormore clock rates, block 950 is modified to determine whether more thanone autocorrelation output indicates presence of a carrier, and block952 is modified to consider a third or more autocorrelation outputs.

FIG. 17 is a flow diagram of an example method 960 for generating a dataunit according to the second example preamble design (of which anexample embodiment is shown in FIG. 8), according to an embodiment. Insome embodiments, an AP such as the AP 14 of FIG. 1 (and/or a clientstation such as the client station 25-1) is configured to implement themethod 960 to generate a data unit for transmission over a communicationchannel.

At block 962, it is determined whether a PHY mode of a communicationdevice implementing the method 960 is a first PHY mode or a second PHYmode. In various embodiments, block 962 is similar to block 922 ofmethod 920 in FIG. 15.

If it is determined at block 962 that the PHY mode is the first PHYmode, the flow proceeds to block 964. At block 964, a first number ofrepetitions (i.e., one or more repetitions) of a sequence is generatedin a first preamble portion. In some embodiments, the first preambleportion is generated using a first clock rate corresponding to the clockrate of an OFDM portion when in the first PHY mode, while in otherembodiments the first preamble portion is generated using a second clockrate that corresponds to the clock rate of an OFDM portion when in thesecond PHY mode. In an embodiment, the repeated sequences of the firstpreamble portion are OFDM-modulated (e.g., are OFDM-modulated sequencesof an STF, in an embodiment).

At block 968, an OFDM portion is generated using the first clock rate.In various embodiments, block 968 is similar to block 926 of method 920in FIG. 15.

If it is determined at block 962 that the PHY mode is the second PHYmode, the flow proceeds to block 970. At block 970, a second number ofrepetitions of a sequence is generated in a first preamble portion. Thesecond number of repetitions is greater than the first number ofrepetitions generated at block 964, and causes the first preambleportion to be longer than the first preamble portion generated at block964. In an embodiment, each repeating sequence generated at block 970 isthe same as each repeating sequence generated at block 964. For example,in an embodiment, the sequences of the first preamble portion aregenerated at block 970 using the same clock rate as is used to generatethe sequences of the first preamble portion at block 964, and thesequences of the first preamble portions generated at blocks 964 and 970are both OFDM-modulated sequences of an STF.

At block 972, an OFDM portion is generated using the second clock rate.In various embodiments, block 972 is similar to block 932 of method 920in FIG. 15.

In some embodiments, the method 960 of FIG. 17 includes additionalmethod elements not shown. For example, in an embodiment, after block968 and block 972 an additional method element includes transmitting viaa communication channel (e.g., a wireless communication channel) a dataunit that includes both the generated first preamble portion and thegenerated OFDM portion. Moreover, although blocks 968 and 972 are shownlater in the flow diagram of example method 960 than blocks 964 and 970,respectively, blocks 968 and 972 occur before, or simultaneously with,blocks 964 and 970 in other embodiments.

FIG. 18 is a flow diagram of an example method 980 for auto-detecting aclock rate of a data unit generated according to the second examplepreamble design (of which an example embodiment is shown in FIG. 8),according to an embodiment. In some embodiments, an AP such as the AP 14of FIG. 1 (and/or a client station such as the client station 25-1) isconfigured to implement the method 980.

At block 982, a data unit is received via a communication channel. Block982 is similar to block 912 of the method 910 in FIG. 14, in someembodiments. The data unit received at block 982 includes a firstpreamble portion, and an OFDM portion that follows the first preambleportion. The OFDM portion of the data unit includes a second preambleportion including one or more LTFs. According to various embodiments,the received data unit is a data unit having the preamble designdescribed in connection with FIG. 8. Additionally or alternatively,according to various embodiments, the received data unit is a data unitgenerated according to the method 960 of FIG. 17.

At block 984, an autocorrelation of at least a first preamble portion ofthe data unit received at block 982 is performed, where theautocorrelation outputs a carrier sense signal.

At block 988, the clock rate of the OFDM portion of the data unitreceived at block 982 is determined based on a pulse length of thecarrier sense signal (i.e., the autocorrelation output). In anembodiment, the clock rate of the OFDM portion is determined to be afirst clock rate if the carrier sense signal is at a “high” level (orany other indicator of a relatively strong autocorrelation) for a firstlength of time (e.g., 0.8 μs), and is determined to be a lower, secondclock rate if the carrier sense signal is at a “high” level (or anyother indicator of a relatively strong autocorrelation) for a longer,second length of time (e.g., 3.2 μs). In some embodiments, thedetermination at block 988 is performed by determining whether a pulselength of the carrier sense signal is in a first length range (e.g.,less than 10 μs) or a second length range (e.g., greater than 10 μs).The pulse length depends on the number of repeated sequences in thefirst preamble portion of the data unit, in an embodiment. In anembodiment, the pulse length of the carrier sense signal (e.g., durationof a pulse in the carrier sense signal) corresponds to an estimation ofa length of time between i) sensing a carrier and ii) detecting atransition from the first preamble portion of the received data unit tothe second preamble portion of the received data unit.

In some embodiments, the method 988 includes additional method elementsnot shown in FIG. 18. For example, in an embodiment, the method 980includes providing, prior to receiving the data unit at block 982, areceiver clock rate that corresponds to the first, higher potentialclock rate of received data units. Moreover, while the method 980 hasbeen described with reference to determining a first and a second clockrate, some embodiments include additional method elements (e.g., similarto block 984) corresponding to a third clock rate, a third and a fourthclock rate, etc., where it is also determined at block 988 whether theclock rate of the OFDM portion is one of these additional potentialclock rates.

FIG. 19 is a flow diagram of an example method 1000 for generating adata unit according to the third example preamble design (of which anexample embodiment is shown in FIG. 9), according to an embodiment. Insome embodiments, an AP such as the AP 14 of FIG. 1 (and/or a clientstation such as the client station 25-1) is configured to implement themethod 1000 to generate a data unit for transmission over acommunication channel.

At block 1002, it is determined whether a PHY mode of a communicationdevice implementing the method 1000 is a first PHY mode or a second PHYmode. In various embodiments, block 1002 is similar to block 922 ofmethod 920 in FIG. 15.

If it is determined at block 1002 that the PHY mode is the first PHYmode, the flow proceeds to block 1004. At block 1004, a first number ofrepetitions (i.e., one or more repetitions) of a sequence (i.e., one ormore repetitions) is generated in a first preamble portion. Block 1004is similar to block 964 in method 960 of FIG. 17, in an embodiment.

At block 1008, the first preamble portion generated at block 1004 isaugmented using a first cover code. For example, in an embodiment, thefirst preamble portion is augmented using a sequence of all ones (i.e.,such that the polarity of all bits in all repeating sequences of thefirst preamble portion is not changed). In one embodiment where thefirst cover code is a sequence of all ones, augmenting the firstpreamble portion at block 1008 comprises simply not performing any covercode processing operation on the first preamble portion. In anembodiment where the first cover code is a sequence of all ones, block1008 is omitted.

At block 1010, an OFDM portion is generated using a first clock ratecorresponding to the first PHY mode. In various embodiments, block 968is similar to block 926 of method 920 in FIG. 15.

If it is determined at block 1002 that the PHY mode is the second PHYmode, the flow proceeds to block 1012. At block 1012, a second number ofrepetitions (i.e., one or more repetitions) of a sequence is generatedin a first preamble portion. In one embodiment, the second number ofrepetitions is the same as the first number of repetitions generated atblock 1004 (i.e., the number of repetitions, and therefore firstpreamble portion length, does not reflect the PHY mode or the clock rateof the OFDM portion). In another embodiment, the second number ofrepetitions is greater than the first number of repetitions generated atblock 1004, and results in the first preamble portion being longer thanthe first preamble portion generated at block 1004. In an embodiment,each repeating sequence generated at block 1012 is the same as eachrepeating sequence generated at block 1004. For example, in anembodiment, the sequences of the first preamble portion are generated atblock 1012 using the same clock rate as is used to generate thesequences of the first preamble portion at block 1004, and the sequencesof the first preamble portions generated at blocks 1004 and 1012 areboth OFDM-modulated sequences of an STF.

At block 1014, the first preamble portion generated at block 1012 isaugmented using a second cover code that is different than the firstcover code utilized for the first PHY mode. For example, in oneembodiment where the first cover code is a sequence of all ones, thefirst preamble portion is augmented at block 1014 using a series ofalternating positive and negative ones (e.g., such that the polarity ofall bits in every second instance of the sequence is changed, in anembodiment). In an embodiment where the second cover code is a sequenceof all ones, augmenting the first preamble portion at block 1014comprises simply not performing any cover code processing operation onthe first preamble portion. In an embodiment where the second cover codeis a sequence of all ones, block 1014 is omitted.

At block 1018, an OFDM portion is generated using a second clock ratecorresponding to the second PHY mode. In various embodiments, block 1018is similar to block 932 of method 920 in FIG. 15.

In some embodiments, the method 1000 of FIG. 19 includes additionalmethod elements not shown. For example, in an embodiment, after block1010 block 1018, an additional method element includes transmitting viaa communication channel (e.g., a wireless communication channel) a dataunit that includes both the generated first preamble portion and thegenerated OFDM portion. Moreover, the sequence of blocks in each path ofthe method 1000 is different in various embodiments, and/or one or moreof the blocks of the method 1000 is performed simultaneously with otherblocks. For example, blocks 1004 and 1008 (or blocks 1012 and 1014)occur after or in parallel with block 1010 (or block 1018), in anembodiment.

FIG. 20 is a flow diagram of an example method 1020 for auto-detecting aclock rate of a data unit generated according to the third examplepreamble design (of which an example embodiment is shown in FIG. 9),according to an embodiment. In some embodiments, an AP such as the AP 14of FIG. 1 (and/or a client station such as the client station 25-1) isconfigured to implement the method 1020.

At block 1022, a data unit is received via a communication channel.Block 1022 is similar to block 912 of the method 910 in FIG. 14, in someembodiments. The data unit received at block 1022 includes a firstpreamble portion, and an OFDM portion that follows the first preambleportion. The OFDM portion of the data unit includes a second preambleportion including one or more LTFs. According to various embodiments,the received data unit is a data unit having the preamble designdescribed in connection with FIG. 9. Additionally or alternatively,according to various embodiments, the received data unit is a data unitgenerated according to the method 1000 of FIG. 19.

At block 1024, at least a first preamble portion of the data unitreceived at block 1022 is processed to remove or undo a first possiblecover code. For example, in an embodiment, a first possible cover codeis a series of ones, utilized by transmitting devices that transmit dataunits in a first PHY mode using a first clock rate. In an embodimentwhere the first possible cover code is a series of ones, block 1024 isomitted.

At block 1028, a first autocorrelation of at least the first preambleportion (as processed at block 1024) is performed. The firstautocorrelation is performed using a first repetition period and outputsa first carrier sense signal. In an embodiment, block 1028 is similar toblock 944 of FIG. 16.

At block 1030, at least the first preamble portion of the data unitreceived at block 1022 is processed to remove or undo a second possiblecover code. For example, in one embodiment where the first possiblecover code is a series of ones utilized by transmitting devices in afirst PHY mode using a first clock rate, a second possible cover code isa series of alternating positive and negative ones utilized bytransmitting devices in a second PHY mode using a second clock ratedifferent than the first clock rate. In an embodiment, block 1030 isperformed in parallel with block 1024. In an embodiment where the secondpossible cover code is a series of ones, block 1030 is omitted.

At block 1032, a second autocorrelation of at least the first preambleportion (as processed at block 1030) is performed, where the secondautocorrelation is performed using a second repetition period andoutputs a second carrier sense signal. In an embodiment, block 1032 issimilar to block 948 of FIG. 16 (e.g., in an embodiment, block 1032 isperformed in parallel with block 1028).

At block 1034, the clock rate of the OFDM portion of the data unitreceived at block 1022 is determined based on which autocorrelationoutput indicates the presence of a carrier. For example, in anembodiment, if the first carrier sense signal output by the firstautocorrelation performed at block 1028 indicates the presence of acarrier (e.g., outputs a “high” level, or otherwise indicates arelatively strong autocorrelation), it is determined that the clock rateis the first clock rate, and if the second carrier sense signal outputby the second autocorrelation performed at block 1032 indicates thepresence of a carrier (e.g., outputs a “high” level, or otherwiseindicates a relatively strong autocorrelation), it is determined thatthe clock rate is the second clock rate.

Because each of the first and second autocorrelations follows processingthat attempts to remove or undo one of the alternative cover codes ofthe preamble design scheme, most likely only one of the first and secondcarrier sense signals will indicate the presence of a carrier. Moreover,this carrier sensing generally occurs near the beginning of a carriersense signal pulse, without having to wait to see the length of thepulse. Accordingly, in an embodiment, the receiver clock is dynamicallyadjusted to correspond to the clock rate of the received data unit basedon the determination at block 1034.

In some embodiments, the method 1020 includes additional method elementsnot shown in FIG. 20, such as dynamically adjusting the receiver clockas described above, for example. Moreover, while the method 1020 hasbeen described with reference to determining a first and a second clockrate, some embodiments include additional method elements (e.g., similarto blocks 1024 and 1028) corresponding to a third clock rate, a thirdand a fourth clock rate, etc., where it is also determined at block 1034whether the clock rate of the OFDM portion is one of these additionalpotential clock rates.

FIG. 21 is a flow diagram of an example method 1040 for generating adata unit according to the fourth or fifth example preamble design (ofwhich example embodiments are shown in FIGS. 10 and 11, respectively),according to an embodiment. In some embodiments, an AP such as the AP 14of FIG. 1 (and/or a client station such as the client station 25-1) isconfigured to implement the method 1040 to generate a data unit fortransmission over a communication channel.

At block 1042, it is determined whether a PHY mode of a communicationdevice implementing the method 1040 is a first PHY mode or a second PHYmode. In various embodiments, block 1042 is similar to block 922 ofmethod 920 in FIG. 15.

If it is determined at block 1042 that the PHY mode is the first PHYmode, the flow proceeds to block 1044. At block 1044, a first SYNC fieldis generated in a first preamble portion, where the first preambleportion is an SC “extra preamble” portion as discussed above. In someembodiments, the first SYNC field includes a repeating sequence (e.g., arepeating Barker sequence, Golay code sequence, etc., according tovarious embodiments). In an embodiment, the first SYNC field is the sameas or substantially similar to a SYNC field conforming to the IEEE802.11b standard.

At block 1048, a start frame delimiter (SFD) field is generated. The SFDfield is included in the first preamble portion and follows the SYNCfield generated at block 1044. In an embodiment, the SFD field is thesame as or substantially similar to an SFD field conforming to the IEEE802.11b standard. The SFD field is clocked at the same rate as the SYNCfield, in an embodiment.

At block 1050, an OFDM portion is generated using a first clock ratecorresponding to the first PHY mode. In various embodiments, block 1050is similar to block 926 of method 920 in FIG. 15.

If it is determined at block 1042 that the PHY mode is the second PHYmode, the flow proceeds to block 1052. At block 1052, a second SYNCfield, different than the first SYNC field generated at block 1044, isgenerated in a first preamble portion. In one embodiment, the secondSYNC field has a length that is different than the length of the firstSYNC field generated at block 1044. Alternatively, in anotherembodiment, the second SYNC field includes a repeating sequence that iscomplementary to a repeating sequence of the first SYNC field. Forexample, in an embodiment, the first and second SYNC fields includecomplementary Golay code sequences. In an embodiment, the second SYNCfield is the same as or substantially similar to a SYNC field confirmingto the IEEE 802.11b standard. Moreover, in an embodiment, the secondSYNC field is clocked at the same rate as the first SYNC field.

At block 1054, an SFD field is generated. The SFD field is included inthe first preamble portion and follows the SYNC field generated at block1052. In one embodiment, the SFD field generated at block 1054 is thesame as the SFD field generated at block 1048. In another embodiment,the SFD field generated at block 1054 is different than the SFD fieldgenerated at block 1048. For example, in an embodiment where the firstand second SYNC fields generated at blocks 1044 and 1052 includecomplementary Golay code sequences Ga and Gb, respectively, the SFDfield generated at block 1048 includes one or more repetitions of Gb andthe SFD field generated at block 1054 includes one or more repetitionsof Ga. The SFD field is clocked at the same rate as the SYNC field, inan embodiment.

At block 1058, an OFDM portion is generated using a second clock ratecorresponding to the second PHY mode. In various embodiments, block 1058is similar to block 926 of method 920 in FIG. 15.

In some embodiments, the method 1040 of FIG. 21 includes additionalmethod elements not shown. For example, in an embodiment, after block1050 and block 1058 an additional method element includes transmittingvia a communication channel (e.g., a wireless communication channel) adata unit that includes both the generated first preamble portion andthe generated OFDM portion. Moreover, the sequence of blocks in eachpath of the method 1040 is different in various embodiments, and/or oneor more of the blocks of the method 1040 is performed simultaneouslywith other blocks For example, blocks 1044 and 1048 (or blocks 1052 and1054) occur after or in parallel with block 1050 (or block 1058), in anembodiment. Further, in some embodiments, blocks 1048 and 1054 areomitted (i.e., the generated first preamble portion, and therefore thegenerated data unit, does not include an SFD regardless of PHY mode).Still further, in some embodiments where blocks 1048 and 1054 areincluded (i.e., an SFD is included in the first preamble portion), theOFDM portion generated at block 1050 and at block 1058 does not includean STF, and an LTF of the OFDM portion immediately follows the SFD ofthe first preamble portion.

FIG. 22 is a flow diagram of an example method 1060 for auto-detecting aclock rate of a data unit generated according to the fourth or fifthexample preamble design (of which example embodiments are shown in FIGS.10 and 11, respectively), according to an embodiment. In someembodiments, an AP such as the AP 14 of FIG. 1 (and/or a client stationsuch as the client station 25-1) is configured to implement the method1060.

At block 1062, a data unit is received via a communication channel.Block 1062 is similar to block 912 of the method 910 in FIG. 14, in someembodiments. The data unit received at block 1062 includes a firstpreamble portion that is an SC “extra preamble” including a SYNC fieldand, in some embodiments, an SFD following the SYNC field. An OFDMportion of the data unit follows the first preamble portion. The OFDMportion of the data unit includes a second preamble portion includingone or more LTFs and, in some embodiments, an STF preceding the LTF(s).According to various embodiments, the received data unit is a data unithaving the preamble design described in connection with FIG. 10 or adata unit having the preamble design described in connection with FIG.11. Additionally or alternatively, according to various embodiments, thereceived data unit is a data unit generated according to the method 1040of FIG. 21.

At block 1064, a clock rate of the OFDM portion of the data unitreceived at block 1062 is auto-detected or determined based on the SYNCfield in the first preamble portion of the data unit. More specifically,it is determined whether the clock rate is a first clock ratecorresponding to a first PHY mode or a lower, second clock ratecorresponding to a second PHY mode, in an embodiment. In variousembodiments, the PHY modes are similar to any of the PHY modes describedin connection with block 902 of method 900 in FIG. 13.

In some embodiments, the clock rate of the OFDM portion is determinedbased on a length of the SYNC field (e.g., when received data unitsconform to the fourth example preamble design of FIG. 10). In otherembodiments, the clock rate of the OFDM portion is determined based onthe repeating sequence of the SYNC field (e.g., when received data unitsconform to the fourth example preamble design of FIG. 11). For example,in an embodiment, the clock rate is determined based on whether a firstGolay code sequence is included in the SYNC field or a second,complementary Golay code sequence is included in the SYNC field. In oneembodiment, where received data units also utilize different SFDs toindicate the clock rate, determining the clock rate at block 1064includes determining the clock rate also based on the SFD of the firstpreamble portion of the received data unit.

Although FIG. 22 shows only blocks 1062 and 1064 in the method 1060,some embodiments include additional method elements. Moreover, while themethod 1060 has been described with reference to determining a first ora second clock rate, some embodiments additionally determine (at block1064) whether the clock rate is a third clock rate, whether the clockrate is a third or fourth clock rate, etc.

FIG. 23 is a flow diagram of an example method 1080 for generating adata unit according to the sixth example preamble design (of which anexample embodiment is shown in FIG. 12), according to an embodiment. Insome embodiments, an AP such as the AP 14 of FIG. 1 (and/or a clientstation such as the client station 25-1) is configured to implement themethod 1080 to generate a data unit for transmission over acommunication channel.

At block 1082, it is determined whether a PHY mode of a communicationdevice implementing the method 1080 is a first PHY mode or a second PHYmode. In various embodiments, block 1082 is similar to block 922 ofmethod 920 in FIG. 15.

If it is determined at block 1082 that the PHY mode is the first PHYmode, the flow proceeds to block 1084. At block 1084, a SYNC field isgenerated in a first preamble portion, where the first preamble portionis an SC “extra preamble” portion as discussed above. In someembodiments, the SYNC field includes a repeating sequence (e.g., arepeating Barker sequence, Golay code sequence, etc., according tovarious embodiments). In an embodiment, the SYNC field is the same as orsubstantially similar to a SYNC field conforming to the IEEE 802.11bstandard.

At block 1088, a first SFD field is generated. The first SFD field isincluded in the first preamble portion and follows the SYNC fieldgenerated at block 1084. In an embodiment, the first SFD field is thesame as or substantially similar to an SFD field conforming to the IEEE802.11b standard. The SFD field is clocked at the same rate as the SYNCfield, in an embodiment.

At block 1090, an OFDM portion is generated using a first clock ratecorresponding to the first PHY mode. In various embodiments, block 1090is similar to block 926 of method 920 in FIG. 15.

If it is determined at block 1082 that the PHY mode is the second PHYmode, the flow proceeds to block 1092. At block 1092, a SYNC field isgenerated in a first preamble portion. The SYNC field is the same as orsubstantially the same as the SYNC field generated at block 1084, in anembodiment.

At block 1094, a second SFD field different than the first SFD field isgenerated. For example, in one embodiment, the second SFD field includesa sequence that is repeated in the SYNC field but with a sign flip,while the first SFD field includes the same sequence of the SYNC fieldwithout the sign flip. As another example, in an embodiment, the secondSFD field includes one or more repetitions of a sequence that isdifferent than a sequence repeated one or more times in the first SFDfield. The second SFD field is included in the first preamble portionand follows the SYNC field generated at block 1092. The second SFD fieldis clocked at the same rate as the SYNC field, in an embodiment.

At block 1098, an OFDM portion is generated using a second clock ratecorresponding to the second PHY mode. In various embodiments, block 1098is similar to block 926 of method 920 in FIG. 15.

In some embodiments, the method 1080 of FIG. 23 includes additionalmethod elements not shown. For example, in an embodiment, after block1090 and block 1098 an additional method element includes transmittingvia a communication channel (e.g., a wireless communication channel) adata unit that includes both the generated first preamble portion andthe generated OFDM portion. Moreover, the sequence of blocks in eachpath of the method 1080 is different in various embodiments, and/or oneor more of the blocks of the method 1080 is performed simultaneouslywith other blocks. For example, blocks 1084 and 1088 (or blocks 1092 and1094) occur after or simultaneously with block 1090 (or block 1098), inan embodiment. In some embodiments, the OFDM portion generated at block1090 and at block 1098 does not include an STF, and an LTF of the OFDMportion immediately follows the SFD.

FIG. 24 is a flow diagram of an example method 1100 for auto-detecting aclock rate of a data unit generated according to the sixth examplepreamble design (of which an example embodiment is shown in FIG. 12),according to an embodiment. In some embodiments, an AP such as the AP 14of FIG. 1 (and/or a client station such as the client station 25-1) isconfigured to implement the method 1100.

At block 1102, a data unit is received via a communication channel.Block 1102 is similar to block 912 of the method 910 in FIG. 14, in someembodiments. The data unit received at block 1102 includes a firstpreamble portion that is an SC “extra preamble” including a SYNC fieldand an SFD following the SYNC field. An OFDM portion of the data unitfollows the first preamble portion. The OFDM portion of the data unitincludes a second preamble portion including one or more LTFs and, insome embodiments, an STF preceding the LTF(s). In some embodiments,however, the second preamble portion does not include an STF, and theLTF(s) immediately follow the SFD of the first preamble portion.According to various embodiments, the received data unit is a data unithaving the preamble design described in connection with FIG. 12.Additionally or alternatively, according to various embodiments, thereceived data unit is a data unit generated according to the method 1080of FIG. 23.

At block 1104, a clock rate of the OFDM portion of the data unitreceived at block 1102 is auto-detected or determined based on the SFDfield (but not based on the SYNC field) in the first preamble portion ofthe data unit. More specifically, it is determined whether the clockrate is a first clock rate corresponding to a first PHY mode or a lower,second clock rate corresponding to a second PHY mode, in an embodiment.In various embodiments, the PHY modes are similar to any of the PHYmodes described in connection with block 902 of method 900 in FIG. 13.

In some embodiments, the SFD field (and, therefore, the clock rate ofthe OFDM portion) is determined by performing parallelcross-correlations. For example, in an embodiment, a firstcross-correlation correlates a received SFD sequence with a firstpotential SFD sequence corresponding to the first clock rate, and asecond cross-correlation correlates the received SFD sequence with asecond potential SFD sequence corresponding to the second clock rate.

Although FIG. 24 shows only blocks 1102 and 1104 in the method 1100,some embodiments include additional method elements. Moreover, while themethod 1100 has been described with reference to determining a first anda second clock rate, some embodiments additionally determine (at block1104) whether the clock rate is a third clock rate, whether the clockrate is a third or fourth clock rate, etc.

At least some of the various blocks, operations, and techniquesdescribed above may be implemented utilizing hardware, a processorexecuting firmware instructions, a processor executing softwareinstructions, or any combination thereof. When implemented utilizing aprocessor executing software or firmware instructions, the software orfirmware instructions may be stored in any computer readable memory suchas on a magnetic disk, an optical disk, or other storage medium, in aRAM or ROM or flash memory, processor, hard disk drive, optical diskdrive, tape drive, etc. Likewise, the software or firmware instructionsmay be delivered to a user or a system via any known or desired deliverymethod including, for example, on a computer readable disk or othertransportable computer storage mechanism or via communication media.Communication media typically embodies computer readable instructions,data structures, program modules or other data in a modulated datasignal such as a carrier wave or other transport mechanism. The term“modulated data signal” means a signal that has one or more of itscharacteristics set or changed in such a manner as to encode informationin the signal. By way of example, and not limitation, communicationmedia includes wired media such as a wired network or direct-wiredconnection, and wireless media such as acoustic, radio frequency,infrared and other wireless media. Thus, the software or firmwareinstructions may be delivered to a user or a system via a communicationchannel such as a telephone line, a DSL line, a cable television line, afiber optics line, a wireless communication channel, the Internet, etc.(which are viewed as being the same as or interchangeable with providingsuch software via a transportable storage medium). The software orfirmware instructions may include machine readable instructions that,when executed by the processor, cause the processor to perform variousacts.

When implemented in hardware, the hardware may comprise one or more ofdiscrete components, an integrated circuit, an application-specificintegrated circuit (ASIC), etc.

While the present invention has been described with reference tospecific examples, which are intended to be illustrative only and not tobe limiting of the invention, changes, additions and/or deletions may bemade to the disclosed embodiments without departing from the scope ofthe invention.

What is claimed is:
 1. A method for detecting a clock rate of a dataunit, the method comprising: receiving the data unit via a communicationchannel, wherein the data unit includes i) a first preamble portion andii) an orthogonal frequency division multiplexing (OFDM) portionfollowing the first preamble portion, and the OFDM portion includes asecond preamble portion including one or more long training fields; anddetermining, based on the first preamble portion, whether a clock rateof the OFDM portion is a first clock rate or a second clock rate lowerthan the first clock rate.
 2. A method according to claim 1, wherein thefirst preamble portion includes a number of repetitions of a sequence,determining whether the clock rate of the OFDM portion is the firstclock rate or the second clock rate includes performing a firstautocorrelation of at least the first preamble portion using a firstrepetition period and performing a second autocorrelation of at leastthe first preamble portion using a second repetition period longer thanthe first repetition period, and the first autocorrelation outputs afirst carrier sense signal and the second autocorrelation outputs asecond carrier sense signal.
 3. A method according to claim 2, whereindetermining whether the clock rate of the OFDM portion is the firstclock rate or the second clock rate includes when both the first carriersense signal and the second carrier sense signal indicate a presence ofa carrier, determining whether the clock rate of the OFDM portion is thefirst clock rate or the second clock rate based on at least one of i) apulse length of the first carrier sense signal and ii) a pulse length ofthe second carrier sense signal.
 4. A method according to claim 1,wherein the first preamble portion includes a number of repetitions of asequence, determining whether the clock rate of the OFDM portion is thefirst clock rate or the second clock rate i) includes performing anautocorrelation of at least the first preamble portion, and ii) is basedon a pulse length of a carrier sense signal that is output by theautocorrelation, and the pulse length of the carrier sense signaldepends on the number of repetitions of the sequence.
 5. A methodaccording to claim 4, wherein determining whether the clock rate of theOFDM portion is the first clock rate or the second clock rate includesdetermining that the clock rate of the OFDM portion is the first clockrate when the pulse length of the carrier sense signal is in a firstlength range, and determining that the clock rate of the OFDM portion isthe second clock rate when the pulse length of the carrier sense signalis in a second length range.
 6. A method according to claim 1, whereindetermining whether the clock rate of the OFDM portion is the firstclock rate or the second clock rate includes at least one of: (i)processing the first preamble portion to remove a first possible covercode and generate a first processed first preamble portion, andperforming an autocorrelation of at least the first processed firstpreamble portion using a first repetition period, and (ii) processingthe first preamble portion to remove a second possible cover codedifferent than the first possible cover code and generate a secondprocessed first preamble portion, and performing an autocorrelation ofat least the second processed first preamble portion using a secondrepetition period longer than the first repetition period.
 7. A methodaccording to claim 6, wherein the autocorrelation of the first processedfirst preamble portion outputs a first carrier sense signal and theautocorrelation of the second processed first preamble portion outputs asecond carrier sense signal, determining whether the clock rate of theOFDM portion is the first clock rate or the second clock rate is basedon at least one of i) the first carrier sense signal and ii) the secondcarrier sense signal.
 8. A method according to claim 7, furthercomprising: dynamically adjusting a receiver clock based on adetermination of whether the clock rate of the OFDM portion is the firstclock rate or the second clock rate.
 9. A method according to claim 1,wherein the first preamble portion includes a synchronization field, anddetermining whether the clock rate of the OFDM portion is the firstclock rate or the second clock rate is based on the synchronizationfield.
 10. A method according to claim 9, wherein determining whetherthe clock rate of the OFDM portion is the first clock rate or the secondclock rate is based on a length of the synchronization field.
 11. Amethod according to claim 9, wherein determining whether the clock rateof the OFDM portion is the first clock rate or the second clock rateincludes determining that the clock rate of the OFDM portion is thefirst clock rate when the synchronization field includes a firstrepeating sequence, and determining that the clock rate of the OFDMportion is the second clock rate when the synchronization field includesa second repeating sequence.
 12. A method according to claim 11, whereinthe first repeating sequence is a Golay code sequence and the secondrepeating sequence is a complementary Golay code sequence.
 13. A methodaccording to claim 9, wherein the first preamble portion includes astart frame delimiter field, and determining whether the clock rate ofthe OFDM portion is the first clock rate or the second clock rate isfurther based on the start frame delimiter field.
 14. A method accordingto claim 9, wherein the first preamble portion includes a start framedelimiter field, and the one or more long training fields of the secondpreamble portion immediately follow the start frame delimiter field ofthe first preamble portion.
 15. A method according to claim 1, whereinthe first preamble portion includes a synchronization field and a startframe delimiter field, and determining whether the clock rate of theOFDM portion is the first clock rate or the second clock rate is basedon the start frame delimiter field.
 16. A method according to claim 1,wherein determining whether the clock rate of the OFDM portion is thefirst clock rate or the second clock rate further includes determiningwhether the clock rate of the OFDM portion is a third clock rate lowerthan the second clock rate.
 17. An communication device comprising: anetwork interface configured to receive a data unit via a communicationchannel, wherein the data unit includes i) a first preamble portion andii) an orthogonal frequency division multiplexing (OFDM) portionfollowing the first preamble portion, and the OFDM portion includes asecond preamble portion including one or more long training fields; anddetermine, based on the first preamble portion, whether a clock rate ofthe OFDM portion is a first clock rate or a second clock rate lower thanthe first clock rate.
 18. A communication device according to claim 17,wherein the first preamble portion includes a number of repetitions of asequence, and the network interface is configured to determine whetherthe clock rate of the OFDM portion is the first clock rate or the secondclock rate by performing a first autocorrelation of at least the firstpreamble portion using a first repetition period and performing a secondautocorrelation of at least the first preamble portion using a secondrepetition period longer than the first repetition period, wherein thefirst autocorrelation outputs a first carrier sense signal and thesecond autocorrelation outputs a second carrier sense signal, and whenboth the first carrier sense signal and the second carrier sense signalindicate a presence of a carrier, determining whether the clock rate ofthe OFDM portion is the first clock rate or the second clock rate basedon at least one of i) a pulse length of the first carrier sense signaland ii) a pulse length of the second carrier sense signal.
 19. Acommunication device according to claim 17, wherein the first preambleportion includes a number of repetitions of a sequence, and the networkinterface is configured to determine whether the clock rate of the OFDMportion is the first clock rate or the second clock rate by performingan autocorrelation of at least the first preamble portion, wherein theautocorrelation of at least the first preamble portion outputs a carriersense signal having a pulse length that depends on the number ofrepetitions of the sequence, by determining that the clock rate of theOFDM portion is the first clock rate when the pulse length of thecarrier sense signal is in a first length range, and by determining thatthe clock rate of the OFDM portion is the second clock rate when thepulse length of the carrier sense signal is in a second length range.20. A communication device according to claim 17, wherein the networkinterface is configured to determine whether the clock rate of the OFDMportion is the first clock rate or the second clock rate by at least oneof: (i) processing the first preamble portion to remove a first possiblecover code and generate a first processed first preamble portion, andperforming an autocorrelation of at least the first processed firstpreamble portion using a first repetition period, and (ii) processingthe first preamble portion to remove a second possible cover codedifferent than the first possible cover code and generate a secondprocessed first preamble portion, and performing an autocorrelation ofat least the second processed first preamble portion using a secondrepetition period longer than the first repetition period.
 21. Acommunication device according to claim 17, wherein the first preambleportion includes a synchronization field, and the network interface isconfigured to determine whether the clock rate of the OFDM portion isthe first clock rate or the second clock rate based on a length of thesynchronization field.
 22. A communication device according to claim 17,wherein the first preamble portion includes a synchronization field, andthe network interface is configured to determine whether the clock rateof the OFDM portion is the first clock rate or the second clock rate bydetermining that the clock rate of the OFDM portion is the first clockrate when the synchronization field includes a first repeating sequence,and determining that the clock rate of the OFDM portion is the secondclock rate when the synchronization field includes a second repeatingsequence.
 23. A communication device according to claim 17, wherein thefirst preamble portion includes a synchronization field and a startframe delimiter field, and the network interface is configured todetermine whether the clock rate of the OFDM portion is the first clockrate or the second clock rate based on the start frame delimiter field.